It is extremely difficult to keep unaltered males from females in heat. You have to be constantly vigilant during her entire cycle because, as already stated, pregnancy isn't the only concern. They can hurt each other. If it were me, I would alter at least one of them. That way you can still breed the other if and when you are prepared and knowledgable. As LS said, if you are serious about breeding you have a long way to go (health testing, pedigree research, learning how to whelp and raise pups, financially preparing for a likely c-section, etc). Right now, your goal should be keeping your dogs separated and healthy. Good luck.