There is a split-cage method that you use when introducing 2 unfamiliar gerbils that might also work with rats. What you do is that you separate the cage in half with a screen (or u could use chicken wire) and you let them sniff eachother through the screen. Then every day you can change the sides of the rats to that they can smell the other rats scent and get used to it. I think you continue this for about a week or however long it takes for them to get used to eachother. Good Luck!