I'd suggest they both get fixed because when they go into heat they (males) will mark by scratching or peeing in walls and furniture.
That said, I grew up around dwts all my life. A couple years ago we got a she kitty when she got pregnant she had her first litter that were all born and raised indoors. When they got to about 3 weeks old she escaped our home and we found out she was pregnant before we could get her fixed as we planned. Her first litter were about 4 months old by the time the new litter came along. They too were born and raised indoors for the first 3 weeks. We had given up all but 3 of the first litter up for adoption. Their first visit with the newbies they hissed and avoided the babies. But once the newbies were old enough to romp around and play they were welcomed.
All this to say...your kitties will be fine. They'll resolve any disagreements on their own.