They've studied life-sized models of the patients' heads, pored over animated graphics showing the intricacies of their brains and rehearsed surgical strategies that will either make medical history or lead to a family tragedy.
The international team of doctors that will attempt to separate adult twins in Singapore this week has spent months planning the delicate, dangerous operation that could take from 24 hours to several days.
The surgery to detach Ladan and Laleh Bijani - 29-year-old lawyers from Iran who say they can no longer bear to live fused together - is not just a testimony to the possibilities of modern medicine. It is also a feat of engineering and an ordeal of endurance.
"It's about as extraordinary as you can get in surgery," said Dr. J. Alex Haller, a retired pediatric surgeon at the Johns Hopkins Children's Center who separated baby girls conjoined at the abdomen in 1982.
The manpower requirements alone are enormous.
The operation at Singapore's Raffles Hospital is expected to involve 12 surgeons and as many as 100 nurses and assistants. Though some will work in shifts or be outside the operating room, 50 people could be packed into the OR at a time, compared with six to eight during a less complicated procedure.
Doctors will have to have two sets of anesthesia equipment - one for each twin - as well as backups for the 100 or so sterilized instruments they'll use. To help guide them through the surgery, they'll employ a three-dimensional "wand" that pinpoints on a screen where they are in the brain, like a shopping mall map that declares "You are here."
The operation, though, is about more than sophisticated machines and technical skill. It requires the same sort of physical and emotional stamina that defines successful athletes. Though they say they barely notice, surgeons often have to stand or go without sleep for stretches that seem impossibly long - at least to someone who has never completed a grueling medical residency.
"Are they superhuman? No," said Haller. "They're trained, just like a marathon runner. ... With training and experience, they are able to tolerate that."
Even the most painstaking and thorough preparation won't guarantee success; doctors estimate that there is only a 50 percent chance that both twins will survive, and they could emerge brain-damaged. Even well-rested, focused surgeons who have gone through multiple dress rehearsals might find, once they begin operating, that their plan won't work.
"We won't know until we get there," said Dr. Benjamin S. Carson, a pediatric neurosurgeon at the Hopkins Children's Center who is leaving Wednesday for Singapore to join the Bijani sisters' medical team. "We could run into a situation that tells us this can't be done."
Carson, who has separated three sets of twins conjoined at the head, will co-lead the team with Dr. Keith Goh, the neurosurgeon who separated Nepalese twins, also joined at the head, at Singapore General Hospital two years ago.
That case, which attracted worldwide attention, also drew the interest of Ladan and Laleh Bijani, who have long sought separation surgery. The twins, who have distinct bodies, brains and desires in life, were told by German physicians seven years ago that it would be too risky because they share a critical blood vessel that is the main drainage system of the brain.
Now, doctors working with the Bijani sisters are drawing upon their experience separating the girls from Nepal, who turned 3 in May.
6 months to plan
Planning for that operation took six months. The twins, Ganga and Jamuna Shrestha, underwent a battery of tests, including CT scans and MRIs, so surgeons could understand the anatomy of their brains and how they were connected. Physicians rehearsed surgical approaches using computerized simulations and 3-D models of the girls' skull, scalps, brains and blood vessels.
Goh, who trained in Singapore and at Beth Israel Medical Center in New York, set up a teleconference with Carson and others at Hopkins. In the weeks before the operation, the team in Singapore went through several practice runs in the OR.
The surgery, which was expected to take 36 hours, lasted more than 90. Fatigued doctors worked for long stretches, then handed off their work to colleagues and tried to get a few hours of sleep, according to an account in Asiaweek magazine.
A nearby refrigerator was kept stocked. Lots of coffee and bird's nest soup, a Chinese delicacy, were on hand. The wife of one of the medical team's lead doctors reportedly brought in ginseng - said to relieve stress and boost energy levels - and the chief of the hospital prepared breakfast.
Total operating time for Jamuna was 94 hours; for her sister, who required more extensive scalp reconstruction, it was 102.
Time passes unheeded
Carson said physicians who perform long surgeries aren't generally aware of the passage of time - or much else, for that matter. They are too focused on the urgent task at hand.
"It's like being in a jungle with a hungry tiger," he said.