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Though they share many organs, including a single large liver, a bladder, intestines and a reproductive tract, their nervous systems are distinct. Tickle Abby on her side anywhere from head to toe, and Britty can't feel it--except along a narrow region on their back where they seem to share sensation. The girls experience separate hungers and separate urges to urinate and sleep.
The fact that they learned to walk at 15 months seems a miracle of determination, encouragement and teamwork. "We praised them so much," remembers Nancy Oltrogge, the twins' day-care provider, who presided over the process. No one ever instructed the girls about who should move which foot when. "They knew what to do," marvels Oltrogge. "We just had to make sure we watched them because they were a little bit top-heavy and could tip over." Occasionally, though, the twins would disagree on which way to go. "All of a sudden," says Oltrogge, "they're going in circles." The twins have graduated to swimming and riding a bike.
No one can say how two separate brains can synchronize such complex motions. It is possible that the girls have developed an unconscious awareness of the placement each other's limbs. "How do they coordinate upper-body motion like clapping hands?" asks Westerdahl. "I don't know if we can ever answer that."
The idea of separating the twins was dismissed by both parents right from the start, when doctors said there was little chance that both could survive the procedure. "How could you pick between the two?" asks Mike. Even if separation were possible, Patty, as a nurse, could picture all too vividly a pathway of pain, multiple surgeries and lives spent mostly in wheelchairs. "If they were separated, they would pretty much cut them right down the middle. You can see that," she says.
This view is supported by Dr. Benjamin Carson, chief of pediatric neurosurgery at Johns Hopkins Children's Center in Baltimore, Maryland, who has helped separate other twins. "If we were to separate them, we would basically take a couple of individuals who are mobile and change them into invalids." He doubts that both could survive.
Perhaps the closest case in which separation was attempted is that of Eilish and Katie Holton of Ireland. Born in a configuration similar to the Hensels' but with four arms rather than three, the Holton twins were separated in 1992 at age 3 in a 15-hour operation involving 25 doctors at London's Great Ormond Street Children's Hospital. Katie died of heart problems four days later. Eilish survives and hobbles around quite nimbly with an artificial leg. Eilish and her parents visited the Hensels in December 1994. For each family, the visit, recorded for ABC's 20/20, was a stunning encounter with the road not traveled.
Patty and Mike worry about what will happen when the girls enter adolescence. "It's going to be tough on them," Mike suspects. Should there come a point where the girls insist on being separated, says Carson, the possibility could be explored, though conjoined twins have never been successfully divided after early childhood. "They would have to say, 'We can't stand this anymore.'" Aside from the physical difficulty, such a separation, he says, would present a "major emotional trauma."