They were separated a little over a month in different cages. We used the same breeder so the vet said that allowed us to shorten the quarantine time down to 2-3 weeks. Once that was done we swapped toys and huts between the cages for a while. Then bunches of used but non-filthy bedding. We have a Critter Nation plus an extension, so they were stacked on top of each other and separated by a pan. They couldn't see each other but they were aware that other ratties were around.

Finally, moment of truth! We got a new smaller cage as neutral ground which was empty other than bedding, water, and a dish of treats. We put in all four and they were doing great for about ten minutes, but then social order formed. Two were totally submissive but the other two started tussling quite a bit. It was hard to watch them fight, but we couldn't see any blood. I read its ok to let them scrap a little. However, I had to intervene a few times with a glove on my hand when I felt like it could become too much. They gradually calmed down over a couple hours and returned to their own cages. We put them back together the next day and there wasn't much conflict. In fact, they slept in a big pile for most the day so by that evening I felt like combining their cages and hoping for the best. Turns out (aside from one small tiff) they seem to be loving it! I've just left out any accessories that they can trap each other in, such as huts, just in case a fight does happen. Still, I'm staying alert to any signs of fighting.

Sorry for the long response, looking at this now I realize it was quite a lengthy process. It was fun though!