The best method of introducing rats is the carrier method
- which means essentially putting them into a small carrier together for at least a day, possibly also a night as well or longer if necessary, until the are relaxed and snoozing in a heap. Then move into a neutral small bare cage (e.g. a cheap hamster cage, look for a free/cheap one on local sites if you don't have one), and continue through the other stages. Don't rush any of them - the rats need to be bored and cramped in order to bond.
They may remember each other, and they may establish a new hierarchy quickly, or they may not. You haven't said how old they are, nor how long they've been apart, so it's hard to predict.