My boyfriend and I are going to get engaged soon, and when we do, we're going to get a house together. We each have two cats. Does anyone have ideas on how to get them accustomed to each other so we don't have a bunch of fighting? 4 boys...
Are they all neutered? That will help for starters.
Keep your two cats and his two cats separated for about a week, but make sure they know there are other cats around. They will probably sniff each other through the door. Slowly, start feeding them near each other in the same room and just move the bowls closer and closer to each other. Keep an eye on their interactions for a while. There will be hissing but that shouldn't discourage you, it's just how cats communicate and they have to figure out a new hierarchy among them. There are lots of little tips you can try, like putting vanilla extract under their chins so they all smell the same to each other. If you do a google search on "cat introductions", you will find more tips. The big thing with cat introductions is time.
Everything that Sakura said is right on the money. I just wanted to add that both pairs of cats have probably already perceived the scents of the other pair plenty of times, since presumably you and your boyfriend interact with all four cats. So that will help. The cats will all be under the same roof for the first time, but the smells will at least be somewhat familiar.
Also, males tend to more readily accept new cats, so that's another plus. That doesn't necessarily mean that there won't be a tense initial adjustment period, though.
You might want to look into Feliway and/or Rescue Remedy as well.