The most important thing is that they spend the first week separated, so you rule out that the newcomer does not have any disease that can happen to the other. Once the “quarantine period” is over when you put them together the first time, try to keep them in neutral ground and not where they are normal. It may be the sofa, but put some towels in case someone misses the pee! At first they are likely to be ignored, and then they will start to smell and maybe one chases the other. Leave them for a little while and return each one to his cage. Join them again a couple of times. If you see that they get along, you can put them together, but first you must clean the cage where they live, and all the accessories so that it does not smell like any and avoid territoriality problems.