I think you raise a good parallel with Earth but I don't think it applies directly to the possibilities for a multiverse because we cannot see other universes but we can see other planets.

Imagine if our solar system was inside a nebula and the only star we could see was Sol. Until we figure a way to send a probe outside the nebula we have no idea if their are more stars or not. Do you see a way to "send a probe outside" and see if there are more universes or not?