A twice exceptional child (also called 2e) is a child with distinct skill or capabilities while also having a distinguished disability or disorder (usually cognitive).
A child with severe autism who can play the piano like a historical composer would be considered a 2e. There are varying degrees of 2e depending on the level of the skills or abilities compared to the incumberance they live with.
Recently I found a question regarding research of 2e children. Research is mostly geared toward the medical condition than the abilities they may have. For the most part, especially for autism, they aren't sure why some of our spectrum kids become savants.
I have my own theory. I think the brain is compensating. Much like what the phenomenon of the blind having better hearing in some cases or the deaf being more sensitive to other vibrations. I think the compensation factor could apply for about anything the brain is capable of.
Asperger's children have been referred to as "little professors" because most of them get attached to a "favorite subject" of some kind. They can tell you the most trivial points of that subject, but have difficulty changing discussion topics away from the favorite. So, if our Asperger's child likes cars and automotive, he will be "stuck in a rut" talking about cars with difficulty changing to talking about anything else. That doesn't mean he can't change that behavior with practice and support. He can, but it will take time and is best started on early.
There's no doubt that these are amazing children. The following are some videos with examples of these incredible kids. Enjoy.