It was a comic (not hentai, western style) about 2 demon sisters (?) with purple-ish skin iirc. Basically they could sort of absorb one another (maybe through anal vore?) to make their asses bigger. I don't remember much of it, I think it was a series of short comics, and in one part one of them had a date with a boy afaik, and the other one was playing on a hand-held, and she had to force her into her ass or something like that to prepare for her date. In another part you could also see them getting summoned.