kenneth salyer.e in dallas everyone hoped and many people expected the finest doctors, the best machines, and generous financing would all combine to make sense of the impossible choice. but time and tests revealed an outlook which was even more complicated than anybody had imagined. >> this is an orchestration of the most complex surgical procedure that occurs on this earth. there's no question about that in my mind. >> these are the images that confront dr. salyer. a tangled confusion of blood vessels he's never seen before. major veins that originate in one twin, snaking their way deep into the other twin's head. mohamed and ahmed are not only stuck firmly together, they depend on each other for their lives. mohamed, the smaller but more outgoing twin, has more delicate, narrower veins in his head than his brother, and salyer is forced to concede he is the more vulnerable in surgery. surgery they're not even sure is possible. >> as we've moved through it, we've gone from it's doable, it's not doab