Five-month-old Alex and Angel Mendoza weigh 12 pounds each and are joined at the stomach. They share a liver.
Lead surgeon Dr. Stuart Lacey told Early Show co-anchor Julie Chen Thursday, "We CAN separate them," because each has the parts of the liver critical for survival, and each has his own other internal organs.
Early Show correspondent Debbye Turner Bell is at Phoenix Children's Hospital, where the surgery will take place.
A team of nearly two dozen pediatric specialists was to attempt to navigate the complex and delicate surgery.
She got to hold Alex and Angel and says they were "doing great" heading into the surgery.
"They're active, playful and adorable," Bell says.
"Those babies are impossible not to love," says Lacey.
"They're constantly touching each, holding each others' hands," Bell says. "I understand they even suck each other's thumbs. So maybe one of their biggest challenges after the surgery will be getting accustomed to being apart."
Their mother, Ashley Frank, is both hopeful and fearful. She told Bell, "Dr. Lacey is really good and he keeps me reassured that everything is gonna be fine, but as a mom, you still have that, 'What if something goes wrong?' kind of thing."
Nurse Kathy Abbott says she is "excited, nervous, hopeful. ... We've watched them grow over the last (five) months. ... They're ready to be their own, two separate people."
Lacey told Turner he'll consider the operation a success "when they graduate from high school."
"I'm slow to celebrate," he understated.
Births of conjoined twins are very rare, Turner points out. "It only happens one-in-every-200,000 births, which of course makes this surgery quite rare. The babies have already undergone a number of surgical procedures to prepare them for this big day."
Lacey admitted to Chen that, "With all of the media coverage and all of the concern and national interest, there is, of course, some nervousness associated with this. But ... I'll be just focusing on the surgery and what needs to be done for these beautiful little boys."
He told Chen that, while he's never performed this specific type of operation, "All of the different components that are involved in the surgery are things that we do on a regular basis. And by 'We,' I mean the very large team of other physicians, nurses, operating room technicians and other personnel involved in their care. So, although this specific surgery is unique in itself, all of the different components are things that we deal with on a regular basis."
The twins have huge hurdles even after the separation surgery, Lacey added, pointing out that, "One of the biggest challenges both today and in their future is the coverage of their abdomen and chest because since they're fused in that area, there is tissue missing. In other words, skin and muscle that should be there, isn't. So we have to close over those areas using a number of complicated techniques. And that will be an ongoing challenge for them because there will probably be a series of operations before we accomplish our final goal."
And will the operation really take a full day? "We always prepare for the worst or for the longest, but are hopeful that it will be less time than that," Lacey said. "There are so many variables in this case that it will be difficult to know just how long things are going to take. We want to be as expedient and as efficient as we can, but at the same time, we want to be very careful and meticulous."