Hanshin: Half-God(1984)

Hanshin

Conjoined twins Yudy and Yucy are as different as possible. Older Yudy is intelligent but ugly, as her nutrients are used up by her beautiful but simpleminded sister Yucy. Loving and hating so deeply, where does one girl begin and the other end?

Note: Published in English as a part of the "A Drunken Dream and Other Stories" manga anthology by Fantagraphics.