Good question.

I think once the planet is entirely infested, the Xenomorphs have the upper hand. Provided they can still incubate a Necromorph's host. I think the acid would make it impossible for the Necromorphs to do much with the slain Xenomorphs. I think it really comes down to whatever the original life forms were on said planet. Though my dead space knowledge is a little lacking. The Necromorphs might have traits I don't know about. have they proven to be acid resistant?