Maybe mention when you're inviting each one that the other will be invited, too. If the invitee replies, "Oh, I really don't like X, I'll just have to avoid her," I'd think you'd be okay. If she replies, "Oh, you'd better keep her away from me, or I'm likely to get all ignorant," I'd just say, "Oh, then it's probably best for you not to come after all, if you don't think you can be in the same room as her and stay civil." If they BOTH say something along the lines of "I'm not coming if so-and-so is" better for NEITHER to get invited.
Now, if they both sound like they'll be behaved when you inform them of your guest list, and one does her best to avoid the other, but one goes out of her way to try to start something, then you'd know which one not to invite to your next party. Of course, that does run the risk of drama at this party.
Sorry, I don't have any great solutions...just spitballin' here.