So is the male alone during the time you have two females together?
Rats need to have partners at all times. They just don't do well by themselves and can become depressed and sick.
If someone is just starting out breeding, it's a good idea to keep them by themselves to nurse to ensure that there is no tug of war between mothers, both are doing their fare share, and if one decides to eat the litter, your not losing all of them. At least until the rats are 2 weeks of age, before then they are very young and fragile and wont live with those kinds of stresses.