keeping whole litters

I never kept whole litters because of time and kennel limitations. In the past I tried to keep at least 3 that showed me something that I liked as a puppies. Last year I kept 2 males. At about 3 months they started
fighting. I was thinking one would become dominant and thought they would settle it. They were fine outside the pen, but in the pen it wasn't. They would be napping side by side like buddy pals and wake up and look at each other and just tie a$$ fighting for about 5 seconds ending in a draw. It would always end in a draw.
I sold one and then they both were fine. No dog aggression out of either one after that.

But yes that would be the ticket to keep whole litter to weed through them and keep the best.