If their surgical separation is a success, conjoined twins Ladan and Laleh Bijani will have to re-learn basic skills such as holding up their heads and walking fully upright.
But the 29-year-old sisters from Iran will also be forced to confront another aspect of life that they know nothing about: what it's like to be alone.
Separation surgeries are usually performed when conjoined twins are very young, before they become fully aware of themselves, their rare relationship and the possibility that one might not survive the risky ordeal.
"These twins will be the [oldest] twins ever separated, and it will be a toss-up to say how they're going to feel about it psychologically," said Alice D. Dreger, a historian of anatomy at Michigan State University who has studied people with unusual anatomies.
"I think it may well be psychologically easier on younger children because they have not developed a sophisticated understanding of their identities as conjoined people."
Doctors, including Johns Hopkins neurosurgeon Benjamin S. Carson, plan to begin separation surgery of the twins today in Singapore. The operation is expected to take several days.
The sisters, who have separate brains and bodies but are attached at the backs and sides of their heads, have said they are desperate for physical independence.
A physical separation, by definition, means something of an emotional separation as well - one that experts say could leave them feeling disoriented and lonely even while achieving their long-desired freedom.
After all, during their nearly three decades, the identical twin sisters have never slept, bathed, studied, met friends or gone anywhere on their own.
"None of us really knows a whole lot about what that feels like," said Carson. "If things work out well here, I think they will be the subject of significant psychoanalysis."
Even if the sisters defy the odds - the survival rate is only 50 percent - and both emerge healthy from the operation at Singapore's Raffles Hospital, the impact of the separation could be profound.
"These women are likely to experience a level of loneliness when they're apart that others wouldn't feel as intensely," said Dreger.
It hasn't been unusual for twins who lived fused together for even a short time to experience some type of loss - even though they were too young to verbalize it.
After the separation at UCLA last summer of 1-year-old twin girls conjoined at the head, Dr. Henry Kawamoto, the lead plastic and reconstructive surgeon, thought one of them was trying to pull out the sterile dressing covering her head. A nurse in the intensive care unit, however, believed the baby was feeling around for her twin.
"She was waving her hands up at the top [of her head], saying, `What's going on here? There's nobody there,'" Kawamoto said.
Twins Shawna and Janelle Roderick spent just a month fused together at the abdomen before surgeons detached them at Loma Linda University Children's Hospital in California in 1996.
But when the girls were placed in enclosed beds on opposite sides of a hospital room for the first few days after the separation, they often were agitated. They fussed and cried when they were left alone; they seemed comforted only when a nurse sat right in front of them.
"They were experiencing a little bit of anxiety because the face was gone. Something was missing," said the twins' mother, Michelle Roderick, 36, who teaches elementary school science in Prescott, Ariz.
To help ease their daughters' transition to independence, she and her husband, Jeff, bought two dolls and placed one in each girl's bed.
Like many twins, Shawna and Janelle, now healthy 7-year-olds, have different personalities. But they're constantly on the lookout for each other - to the point that one will ask about her sister even if she has only gone to the grocery store. They sometimes pretend they are still connected, hanging on to each other in the position in which they were born, but they have told their parents they are glad they were separated.
"They're still close, but I don't think they need to be physically together to experience that closeness," said Michelle Roderick, who with her father co-founded a support group for conjoined twins and their families called Conjoined Twins International.
In her book Entwined Lives, twin expert Nancy L. Segal describes twins' special relationship - something she calls "friendship extraordinaire." Bonds between twins, especially identical twins, are generally stronger than those between other siblings. And many who have lost their other "half" compare it to losing a part of themselves.
So-called twinless twins often feel like a living reminder of their late sibling, writes Segal, a professor of psychology and director of the Twin Studies Center at California State University, Fullerton, and a twin herself. They might dread the approach of their once-shared birthday or making decisions on their own or question their status as a twin.