Gozu

Demon brothers Kinka and Ginka are conjoined twins whose entire existence is devoted to battling one another to the death. Their species of demon are all born conjoined and immediately after birth the strongest of the pair slays the weaker sibling and the bond between the two is severed. Kinka and Ginka are unusual in that they have both matured to adulthood but have been unable to slay one another, though not through lack of trying