Well, everything has a beginning and end, and during fusions, two points in the universe move together and the rest of the universe spins until it fits. If two Earths merged, even at different points in time, the most that would occur would be some chaos from the actual merging. If two seperate planets in different parts of space did, however, the universe would descend into chaos and desctruction as both try frantically to fit together, but can't because the points they're joined by aren't equal to each other...in others words, Sinumonas gone screw the PokÉarths up =L