I've just started my small group of 2.2 rats and I intend to give the females each their own tub. One male with one female until the girls look nice and round, then the two boys go into their 'bachelor tub' and the girls have their new litters to keep them company. Once litters are weaned, the girls get a week or so to rest together and then they restart. That's the plan, at least, I'll let you know how it goes.