Guide to introducing rats:
-Quarantine the new rat(s) in a separate airspace for 14 days. If either set of rats is sick, this will give time for symptoms to show up without risking the other rats’ health. There are only a few circumstances where a quarantine should be shortened - consult the server for advice. In this case, be prepared for possible illness and vet bills.
-Prepare a neutral area. This can be a bathtub, sofa, or bed. Sprinkle some treats and add the rats. This lets rats meet each other on even ground, where neither of them will feel territorial. This intro should last at least 30 minutes. Squabbling, chasing, squeaking, and some wrestling are all normal. Only separate if you see blood, or if it has been a long time (45+ minutes) and the rats are still significantly distressed and panicked. You can do just one or multiple neutral introductions until the rats are comfortable with each other.
-When the rats are more comfortable (cuddling, eating side by side, etc.), deep clean the cage all the rats will live in. The cage should only contain food, water, and litter boxes. Put all the rats in and monitor. Do not leave them overnight at this point; you need to monitor them for at least several hours. Some more squabbling at this point is normal. However, they should calm down soon enough.
-Every 2-12 hours of settled behavior add an item. Stick to open baskets, climbing/chewing items, flat hammocks, and hides with multiple openings. They may argue again each time you add something. Let them sort through this.
Common misconceptions and other info:
-You do not need to put the rat cages near each other. Rats will be able to smell each other but not interact; stressing them out. Swapping cage items is equally stressful.
-You do not need to put vanilla, baby food, or anything else on the rats. Rats can smell each other through the food; it’s simply unnecessary.
-Carrier method is only needed if rats refuse to interact AND are not aggressive.
Copied this from a server I am in. I hope this helps!