how did we breed after the fusion ? and why can't we bread with chimps?
There are plenty of people today with balanced Robertsonian translocations, they breed mostly fine, though Down's is more common than in the rest of the population, because meiosis is more likely to be messed up. If due to drift, people with those translocations became more common, eventually they could breed with each other, and make offspring with 22 pairs of chromosomes.  In theory, this drift could take over the whole human population.