Meet Abigail and Brittany Hensel, a pair of dicephalic conjoined twins.
When the twins were born, their parents chose not to have them separated. As they share many bodily functions, the operation would have been risky and left them in wheelchairs. They do not regret their decision; the twins have themselves stated that they don't wish to be separated.
Most of Abigail's and Brittany's shared organs are located below the waist line. Together the Hensel twins have:
2 arms—originally 3, but the short malformed central arm between the heads was removed
2 spinal cords and backbones—surgery corrected scoliosis
3½ lungs—surgery expanded their chest cavity while correcting scoliosis
2 hearts in a shared circulatory system—medicine taken by either affects both
1 large intestine
1 reproductive system
Each of the twins manages one side of their conjoined body. By coordinating their efforts, they are able to walk and run normally. They enjoy hobbies and sports including volleyball, basketball, kickball, swimming, cycling, and singing. They also play the piano and are avid computer users. Like many teenagers of their generation, they enjoy softball, digital photography, the internet, social networking, and talking on the telephone.
Despite sharing a body, the twins have different tastes in food and clothes. Some of their clothes are altered so that they have two separate necklines in order to emphasize their individuality. According to a TLC documentary in 2006, they negotiate what they will wear each day. They will usually have separate meals, but sometimes will share a single meal for the sake of convenience (e.g., each takes a bite of a hamburger).
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."
They also appeared on The Oprah Winfrey Show, Life magazine twice, and on The Learning Channel around the time of their 16th birthday, in which they discuss dealing with puberty and getting their driver's licenses.
They successfully passed their driver license exams. They had to take the tests twice, once for each twin. Both control the steering wheel, Abby controls the pedals, transmission, radio, heat, defogger etc., and Brittany controls the turn signal and lights.
More images and a video clip here.
A fascinating article on the girls' amazing lives (parts of which were posted here) by Time entitled The Most Intimate Bond.
The Wikipedia article that makes up most of this post here.
X-posted to wtf_nature.