I don't see why differing spacetime geometries or topologies would impact other worlds. What makes gravity/geometry leak through when nothing else can?
Standard QFT is a fixed background spacetime theory, so if you have multiple non-interacting blobs of probability density in the same spacetime, they will all cooperatively curve it, hence the leakage. If you assert that the spacetime itself splits, you better provide a viable quantum gravity model to show how it happens.
This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous thread is at close to 500 comments.