A) If they're able to interfere, you shouldn't have called them separate worlds in the first place.
B) That's not how interference works. The worlds are constructed to be orthogonal. Therefore, any negative interference in one place will be balanced by positive interference elsewhere, and so you don't end up with less or more than you started with. You don't even need to look at worlds to figure this out - time progression is unitary by the general form of the Schrodinger Equation and the real-valuedness of energy.
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.