They are the rarest of the rarest of the rare.
There is evidence that they can see through each other’s eyes and perhaps share each other’s unspoken thoughts. And if that proves true, it will be the rarest thing of all. They will be unique in the world.
The way their heads are joined, they have markedly different fields of view. One child will look at a toy or a cup. The other can reach across and grab it, even though her own eyes couldn’t possibly see its location.
h/t Mind Hacks