this is "nightline." >> tonight -- >> hi. >> conjoined twin girls, little eva and erika, share one body but with two healthy hearts. ♪ happy birthday to you >> their parents' anguish as one grows stronger and the other weakens. >> you talk to them, you say, you are >> knowing the girls, what they've gone through -- they're fighters. >> tonight the dramatic decision, the risky procedure with two lives on the line. >> a prayer for strength. >> one family's emotional journey. >> was there ever a part of you that thought this might not

work? >> and that moment that brought everyone to tears. >> this christmas "nightline," "becoming two," will be right back. ♪ ♪ connecting people... ...uniting the world.

♪ united. official airline of the golden state warriors. a special christmas "nightline," "becoming two." here's linsey davis. >> reporter: from the moment they were born, you couldn't see

eva sandoval without seeing her sister erika. conjoined twins. they have never known life apart. tonight we take you inside the complicated and risky surgery to separate them. >> have we anticipated all the potential risks? are we prepared to deal with a bad outcome? >> reporter: these two little lives on the line. >> was there ever a part of you that thought this might not work? >> reporter: the sandovals' journey begins in sacramento, california, spring 2014. with children already out of the house, aida and husband art were looking forward to becoming empty nesters. little do they know they're about to receive life-changing news. >> i was having pain. i told art about it. he's like why don't you go to the doctor, i think it's your ulcer. >> reporter: to her surprise she's pregnant, with not one but

two babies. >> so what was your reaction? >> whoa. >> shocked. shock the. i go, okay. >> he was pretty quiet. >> we're not kids anymore. >> reporter: one month later, aida's doctor recommends she see a specialist. she assumes because of her advanced age. >> the doctor came in, he said, we do find some abnormalities. they are twins. he goes, they are conjoined. >> reporter: conjoined twins are a rare phenomenon. their chances of survival even rarer. about half are stillborn. only 35% survive beyond the first day. >> i think when you're hit with some news like that -- he didn't know how to deal with it, i didn't know how to deal with it, you don't know who to turn to. >> reporter: the sandovals reach out to dr. gary hartman at lucille packard children's hospital. >> this is erika's pelvis -- >> reporter: an expert in the world of conjoined twins. despite having had six

successful separation surgeries under his belt, he says every case presents its own challenges. >> what did you say to them then? >> we were pretty blunt. we didn't know that they could be separated. we weren't real optimistic about quality of life. >> reporter: doctors gave them the option of terminating the pregnancy. >> did you ever have a moment where you thought, should we do this? >> i didn't. >> what was it that made you think this is never an option to end this pregnancy? >> i felt it was my faith. >> we talked about it. let's give them a chance, you know. if it's meant to be, it's meant to be. >> reporter: after 33 weeks of pregnancy, the sandovals welcomed two baby girls into the world. erika rose and eva victoria. they are joined from the sternum all the way down to the pelvis, and they share a third leg. but they have two healthy hearts. >> you see them, and they have tubes, they had the covers over their eyes. they did ask us, you can't carry

them, they're very fragile. you question yourself, are we doing the right thing? you talk to them, say you are strong, you're going to get through this. >> reporter: right from the start they begin to defy the odds. but because they need specialized treatment, they must spend the first few months of their lives in the neo natal intensive care unit. >> when those beautiful girls were born, that family was very scared. mom would stand over by the window with the bed many feet away, and we were just working with her to come over and to touch the girls, to know where she could put her hands in a place that wouldn't hurt them. >> reporter: finally, at 7 months old, the girls are deemed strong enough to go home to sacramento. despite aida's 24 years as a mother, nothing could have prepared her for this. >> this is her feeding. we're big her 8 ounces is what she's getting, erika.

>> reporter: even the simplest of tasks, bath time, putting on clothes, become an exercise in patience and creativity. >> these are the little clothes that they wore. these were the little gowns that were made, trying to keep them dressed. something cute is what i wanted to put on them. >> develop cois a godsend, right? >> right. >> reporter: they hit all their first milestones, but they do so together. >> hi! >> reporter: this is from their first words -- >> >> >>p. >> reporter: to learning how to stand. >> very nice. i appreciate that. >> reporter: not wanting to leave their specialized care, they must make the nearly three-hour journey to palo alto regularly for checkups. even during those uncomfortable car rides, the girls remain upbeat. they begin to develop their own

personalities. eva, the talkative one. erika, the observer. eva also becomes stronger and larger than her sister, often dragging, even throwing her around. over time, their health starts to decline. >> what's been the most trying time? >> the utis. they're throwing up, dehydrated. >> eva, stay, don't bounce. >> the month of july, we were in the hospital practically the whole month. >> reporter: erika kept getting weaker and weaker. >> she was just basically getting smaller. eva was getting bigger. so that was their point to say, we really have to do something about this. >> reporter: with concern mounting, the family and doctors decide they need to separate the 2-year-old girls now. >> were you able to explain to them what was going to happen? >> i would always role play. and some magic's going to happen, and dr. hartman is going

to be your magician. >> reporter: having had only 250 separation surgeries successfully performed in the world, doctors tell the sandovals there's a 30% chance one of the twins could die. >> was there a part of you that thought this might not work? >> a very small part. but knowing the girls, what they've gone through, they're fighters. >> reporter: it's finally the morning of the surgery. the twins are their usual bubbly selves. two hours later, when they arrive at the hospital, reality sets in for the sandovals. they place their faith in god and their babies in the hands of this team of specialists. the nearly 50-person medical team begins with a prayer for guidance. in a nearby room the sandovals and dozens of family members

join in. >> our father who art in heaven, hollowed be thy name -- >> i felt a calmness come over me. that it was going to be okay no matter what, what the outcome was. >> reporter: after four hours of careful prep, it's now time to begin the separation. >> they made the first incision. >> reporter: the surgeons' plan is to separate the organs of the chest first, then move down to the abdomen and finish with the pelvis. they're hoping the twins' shared leg can be given to erika. as the hours pass, the doctors encounter a few surprises. >> erika's small intestine joined eva's small intestine just before the large intestine. >> is that a problem? >> actually, you can be fine with just a small intestine. >> reporter: having overcome that challenge, they continue the delicate dance of dividing the girls' organs. >> they were able to split the bladder so they each have a bladder. >> reporter: every step a daunting task.

one false move could spell disaster. >> the big challenge is then closing that wound. that's the real ricky part. when we come back, aida and art are given difficult news. >> it's just like a punch in th >> lonouwa and that moment that brought everyone to tears. ♪ ♪ ♪

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a special christmas "nightline," "becoming two," continues. here again, linsey davis. >> reporter: 2-year-old conjoined twins erika and eva sandoval are undergoing an intense separation surgery.

the past five hours, 13 surgeons at lieu sell packard children hospital have had to carefully coordinate their every move are performing a difficult and deli and then finally -- >> yay! >> reporter: the prayers of their parents, art and aida, are answered. >> yes, wow! >> reporter: the twins, successfully separated. the work of dr. hartman and his team is far from over. >> it seems like the most dramatic piece of the procedure and people are all focused on that, but that's just like step 34 of 50 or 60 steps. >> reporter: the surgical team now faces a new hurdle. they've realized they don't have enough skin to close the girls' wounds. so they turn to that once-shared leg they were hoping to give to erika. it's now needed for something

more crucial, its skin tissue. >> they had told us earlier that they may have been able to use that third leg and give to it erika. when it came down to it, there wasn't enough tissue to cover erika. >> that was really hard. you know, it's just like a punch in the gut. >> reporter: despite that detback, the rest of the surgery goes as planned and after a total of 13 hours, the twins are wheeled into recovery. >> tell me about the first time that you guys saw the girls after the surgery. >> i was excited. just to know they were alive still. just to see them breathing. >> reporter: after spending their first six nights recovering in separate beds, the girls are reunited. in thefoow, erika,

the once smaller twin, is now thriving. she makes tremendous progress at physical therapy. >> she has figured out with this totally new body how to be real mobile really fast. >> reporter: but the road to recovery is not as easy for eva. >> eva, one, two -- there you go. >> with eva, she's had a lot of pain. i think also anxiety. a lot of our sessions are focused on trying to get her in more of an upright position. >> do you want to go see eva? >> reporter: ever her sister's keeper, erika tries to comfort eva.

>> it's okay. it's okay. >> reporter: throught aida is mostly parenting solo. >> with work i could be on the road all week. >> reporter: art, who's had to keep working full-time back at home to cover the medical expenses, drives the three hours every weekend to visit her and the girls. >> i'll do what i have to do to make things work. it's our life. and we adjust to it and keep on going. >> reporter: the hospital becomes like home for the girls, who haven't been outside in two months. >> can you see the clouds moving? what do you see in the clouds? >> elephant. >> you see an elephant in the clouds? >> outside. >> here we go outside. >> mama? >> i'm right here. listen to my voice. >> reporter: finally, after three months in the hospital, a breath of fresh air. an ambulance, while often a sign

of tragedy and fear, on this day is one of hope. >> all right, you made it! listen to you! >> reporter: erika and eva are making the journey home to sacramento. >> you ready to go? >> yeah. >> reporter: in the care of their local hospital, the girls continue physical therapy. >> hold on to your wheelchair. >> reporter: and are fitted for wheelchairs. >> we'll stay right here with you. there you go, see? >> i think it's amazing we're at this point with them in their lives. and feeling sick is not going to hold the other one back. >> reporter: now they're free to roam at home. >> i love it. >> reporter: where for the firs, >> they play so nicely together. >> what is the best part in all of this? >> bringing them home.

being a with every scoop or step, they surpass all expectations. >> good job. you want to move your leg closer. >> reporter: no one was certain they would make it this far. >> this is actually the first that i've seen them both standing up like this. >> these girls are as healthy as they can be right now. they've gone way past what i thought they could do at this time. >> reporter: but they still have many challenges ahead. the girls have been on feeding tubes since bird and are getting used to eating solid foods. they're not able to be fitted for prosthetic legs, but dr. hartman day ty can. >> we prosthesis ght now tha a a completelvis, but nothing, n that side. who knows what we're going to

have in five years or ten years? >> you think walking is not out of the question? >> yeah. watching what they can do now? i think anything is possible. >> reporter: the girls are embracing their independence as typical toddlers full of curiosity and mischief. >> somebody who is supposed to be taking a nap got down from her crib. >> reporter: that connection they once shared never far from memory. >> eva will see her scar and goes, my sister was right here. yes, your sister was right there. we do talk about it. >> she goes, she took my leg! >> she did say that. she took my leg! >> where's your leg? erika took it! ♪ happy birthday to you >> reporter: after two years living as one -- >> are you going to open it? >> reporter: they celebrate their 3rd birthday, their first as separate individuals. >> i'm goi too! >> reporter: at a party full of family and friends --

♪ happy birthday to you >> reporter: the girls dress as woody and princess sofia. even though they're on their own, in some ways they're still inseparable. >> i love watching them. they know that all these people are there and they're going to play with them. whether they have a disability or not, it's just being with people and doing kid stuff. >> are you ready, erika? >> go, eva, come on, go, go go! >> reporter: the girls fearsly fighting to be just like any other kids. >> to do it all over again? >> i would. >> yeah. maybe a little younger. >> i know that god won't give you something you can't handle. there's nothing that these girls can't do. they are fully capable of doing anything. >> i'll have a final note when we come back. ♪

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in recent months, erika and eva have both received prosthetic legs. they continue to surpass expectations. that's "nightline." remember, you can catch all our episodes streaming now on hulu. thanks so much for joining us. good night and merry christmas.