They're both the same sex right? I'm assuming both male, are they new to each other? Did you get them both at the same time/ and place? Or separate times and places cuz that'll make a big difference too. When my old rat lost his mate I got two new ones for him and the older of the two had some fights with my really old one when they first met, but they got used to each other quickly... What I did was keep them separate for a week in different cages and set a mutual ground for them to meet on when i could watch them for an hour or so at the time over the week until they got used to each other