Ohhhhhh boy. Males sometimes do not get along. In fact in some situations they kill each other. If you have an older rat and want to get another, I suggest baby boys. If you have a baby boy, then definetly get another baby boy. Don't get any females, cause although breeding is fun, it's really hard to find the babies homes.
Boys are very territorial. When boys that are strangers are meeting, it's good to have them meet on open unmarked ground. This means that wherever they meet most lack all scents that rats produce. Often times a bed or enclosed table top is fine. Don't give them anything to fight over; food, shelter, your attention, etc. This is all about them meeting each other neutrally.
