E.M.
My dad is a vet, and I have asked him this same question. He says that you can try some prolonged thing where you put one cat in a room, let the other cat get used to the smell, then slowly introduce them for longer and longer periods of time. And then he says that will only make more work for you and maybe feel like you did things "right." In reality, cats are likely to hate each other at first, hiss at each other, and eventually either work it out and be friends or avoid each other. And nothing you do will change that, so just throw 'em together and hope for the best :) He typically has between 4 and 6 cats in his home at any given time.