What sort of conjoined twins would Duane and Belial be?

What sort of conjoined twins would you say Duane and Belial would be? Would such a situation even be possible? There's almost no actual occurance in real life, except "the girl born with two heads", but one "head" was on top of the other. Duane is fully formed, and his identical twin is not, deformed and protruding from his brother's side like a mass, with deformed arms, a remarkably formed and human face, but the rest of him, his organs and some other body structures I would have thought were contained inside Duane--but according to the doctors in the film they only share tissue and flesh. Belial doesn't seem to be able to speak, but he can vocalize and breathe.

There are cases of parasitic twins, with partially formed body structures protruding, but it's never a fully functioning, independantly sentient "head without a body" like Belial.