Although they are two completely separate people, these accomplished teens share a body and have just two arms and legs between them.
Born in 1990, the girls have been brought up in a small, tightly knit community in Minnesota, almost completely protected from prying eyes and inquisitive stares.
To their friends and family, they are distinct people with very different personalities, needs, tastes and desires.
But to the outside world they are a medical mystery – particularly given the fact that they can do virtually all the same things as their friends, including playing the piano, riding a bike, swimming and playing softball. "Their personalities make them inspirational," says their mother Patty.
"They never give up; anything they want to do, they go out and do it." The medical world is keen to find out how two separate brains and nervous systems can work in such a perfectly co-ordinated way, but the twins and their family have always resisted non-essential medical tests.
"The family want to treat them as though they are just like everyone else," says Joy Westerdahl, the girls’ doctor, who admits that it is a mystery how their unique physiology functions.
As they enter adulthood, the twins are likely to leave the haven of their home town and face the wider world. In preparation for that time, they have taken part in this intimate documentary to show the world what it is like to be joined for life.
Watch the full documentary now