It really is hard to say. It depends on the rats involved, your set-up, the babies, and so many other factors.
Do both mothers get along well? Are the babies active? Can you tell which babies belong to who, or do you care if they get mixed up?
Potentially you could introduce both mothers, but if you try it, be extremely careful. They may become possessive and fight with each other over over babies. As the babies get older, introductions will become easier as the mothers become less possessive and desire to get away from the rambunctious babies.