If you want to mate them it's a good idea not to have two females in the same cage while its going on... This is from a site I rely on and ill also give you the link:
-Some pregnant females can live with other females without any problems, but some pregnant females get very protective and may attack their other cage mates.
-Some pregnant rats or new mothers can become selectively aggressive to other rats
-Most females will reach Menopause at around 18 months. (I don't know how old yours are, but that may be helpful?)
There's more in the link, it has everything you need to know about breeding your rats, and mixing two females.
Rat Breeding/Rat Babies
One very important thing is to be prepared for complications that you may need to take your momma to the vet for. And that site is a godsend I hope it helps you :)