I would not put another rat in with the pregnant female because while rats are usually pretty good moms and are unlikely to eat their babies like mice or hamsters when stressed, a 10 gallon is a very tiny area for mom + babies, and you might end up with mom trying to chase off the youngster.

If the younger rat is male (it is very easy to tell... look for testicles... they are massive, but young boys can be good about trying to pull them up insie their bodies) you might end up with him harming the babies, and if it is female, you might end up with her attempting to steal and raise the babies herself -- which is unlikely to go very well.

SO long story short, no don't keep them together. Let mom raise them by herself. If you plan on breeding more, males are capable of getting mom (and sisters) pregnant at 6 weeks, though this is less likely to happenin a 10 gallon tank due to size restraints, and age.

A large female rat can have up to 18 babies, though litters of 6-9 are much more common. Gestation is approximately 21-26 days, and many times female rats will not 'show' a pregnancy until just days before they give birth.

Good luck with it all.