I'm not sure cross-universe communicating is necessary in the multiverse model. The splitting just resembles communication from our perspective in that it makes the probabilities look "rigged".
That still doesn't explain interference patterns in double-slit experiments, especially in double-slit experiments with a single photon/electron at a time. Those can only be explained by the particle/wave traveling through both slits and then the two versions interacting with each other.
If the alternative universes are in different dimensional planes, it's pretty obvious why we couldn't observe them.