Probably fast Sub, there are 'young' children that have a disease that makes them age at an incredible rate, I don't think they even reach the age of 20, because by that time their body would be like 70 years old.
But Xen, are you sure it wasn't just a Siamese twin? One human with two heads, well I have never heard of that. But I have heard of stories of a Siamese twin who shared the exact same body, but they each had their own head.
Suddenly a movie called "Stuck on you" came in my mind. Funny movie. Its about siamese twins that share the same liver.
The second link Zeonix posted was actually about siamese twins too, rather than two heads(Actually theres not much difference).