MWI doesn't posit that the universes are separated. Different classical states of a single quantum computer are technically in different "universes", but are st

MWI doesn't posit that the universes are separated. Different classical states of a single quantum computer are technically in different "universes", but are still interacting with each other. Separation that you are talking about boils down to different parts of wave function being uncorrelated with each other. If this is the case, then events in one "universe" can't have causal connection with another because that would imply correlation.