It's well known that one of human chromosomes is the result of fusion between two chromosomes in a primate ancestor. If we put anthropocentrism aside, it becomes clear that fusion events happened a lot of times, since chromosome count varies in different species.
Yet some people claim that while fusion event is probable, getting it fixed in the population is not, because it would require simultaneous mutation in two animals of opposite sex.
Could anyone explain in layman terms the scenario of this trait's propagation? I may only suppose that even if one parent has two distinct chromosomes and another a single fused chromosome, then two chromosomes may attach to the fused one, and then (probably) get fused themselves, thus leaving fewer and fewer specimens with distinct chromosomes in each generation. Is this a viable and likely scenario, or is some other way of propagation more likely?