." >> 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? the sandovals' journey begins in sacramento, california, in the spring of 2014. with three children almost out of the house, 44-year-old aida sandoval and her husband of 25 years, art, were looking forward to becoming empty nesters. but little do they know they're about to receive some life-changing news. >> i was having pain. then i told art about it. and he's like, go to the doctor, i think it's your ulcer. i said, i don't think it's my ulcer. >> reporter: to her surprise, aida finds out she's pregnant with not one but two babies. so what was your reaction? >> whoa. >> shocked. >> shocked. shocked, like okay -- >> he was pretty quiet. >> we're not

imagine what we can do for you. ♪ ♪ >>> 2-year-old conjoined twins erika and eva sandoval are undergoingtion surgery. the past five hours, 13 surgeons at lucille packard children's hospital have had to carefully coordinate their every move, performing a difficult and delicate dance. and then finally -- >> they're two now. >> reporter: the prayers of their parents art and 8 deaida answered. the twins successfully separated. but 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 realize 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. but when it came down to it, there wasn't enough tissue to cover erika. >> that was really hard. you know, it's just l