Originally Posted by obloquy

I have no idea what you're talking about. They could have coexisted indefinitely just as they coexisted up to the Third Age. Are you imagining the Witch-King packing up his stuff in Morgul and moving into an empty room in Moria? In that case, they both could definitely be expected to get on each others' nerves. Durin's Bane had probably grown accustomed to leaving his underwear laying around and the toilet seat up. The latter of which would truly have ruffled feathers, since, as we all know, the Witch-King sits down to pee.