Plausibly, a fusion of extended objects is extended. Also, plausibly, an extended object has a size. Now suppose, as is surely possible, that there are two universes that aren't spatiotemporally connected, and an extended object A in one and another extended object B in another. Then the fusion of A and B would be an extended object that has no size, since there is no meaningful distance between a part of A and a part of B. Hence, given our assumptions about extended objects, mereological universalism--the thesis that necessarily all pluralities have a fusion--is false.