This post from the Dissection Room over on The Chirurgeon’s Apprentice (an entire blog devoted to “the horrors of pre-anaesthetic surgery”!) is about a condition known as craniopagus parasiticus and the case of an 18th-century Bengali boy:
The normal face and head were not malformed. The brains were distinct, each invested in its own membranes; the dura mater of each adhered to that of the other at the point of contact. The chief supply of blood to the upper head was by a number of vessels passing from the membranes of one brain to that of the other. The movements of the features of the upper head appear to have been purely reflex, and by no means to have been controlled by the feelings or desires of the child. The movements of the eyes of the accessory head did not correspond with those of the child, and the eyelids were usually open, even during sleep.
His skull(s) now reside(s) at the Royal College of Surgeons in London.