I have always kepy my female rats seperate, and rotated my male. Well, I would "rest" the females together, between breedings, in a large tank, and then place the male with them, and then give each girl a solo cage when she became preggo.
With one pair of my females, now, I leave them together. They have raised two litters thus far together; no baby-stealing, baby-eating, or problems. They actually raise the babies together in a big pile. The two were originally sister rats, and have never been seperated.. maybe this is why they get along good?
I have tried pairing females in the past to raise babies, and it always led to one girl stealing all of the babies. I guess it can be done though! I kinda prefer this method, because the females are able to raise their yong with a buddy who can fulfill their need for social interaction. I don't see these girls trying to "escape" like my solo females sometimes do when it seems like they need a break from their young; these females will share motherly duties and this allows each to have a break; but mostly, they're cuddled up with the babies together.