Just let them duke it out.
That's true. When I put a male in with a new female, they fight for awhile, but usually by the end of the day I find them curled up together sleeping. I think a problem may arise if you have a female who's been caged up alone for months and all of a sudden, there's this other rat in her cage. I only buy rats that are "socialized" and already living with other rats, from my local reputable pet store.
I have three breeder females and one "stud" rat who spends his time mating in each of their tanks, usually a week or two at a time. Doesn't he live the good life?