I feel if Haunting of Hill House does happen, I don't see the Conjuring happening. Both are ghosts in a serious tone, so I don't see that happening in the same year. It's not like last year which had Ghostbusters, a horror comedy, with Graveyard Games which was more serious horror.
Also, while I notice some people mentioning IT...I really don't see that happening for one reason or another. The biggest one is the rumor of Jack being the face of marketing. If that ends up true, there's no way they'd have Jack and Pennywise in the same year, plain and simple.