Which leads me to the next issue: BR, RIR and the like need lots of space to roam freely to exhaust their high energy level, or else they develop nasty and aggressive habits such as bullying, feather plucking, egg eating etc.
That's why I let them free range all day long (unless I'm not at home, but I work from home, so I'm almost always there to supervise them). And that's why I was wondering if I could still let them free range if I split them into 2 flocks. I imagine the roosters could have problems if one of them tries to mount a hen from the other's flock... Would letting them out by "turns" help in this case?
From your initial post I assume you are a rather inexperienced chicken owner, and I therefore would strongly advise against keeping both cockerels, as you will probably be apalled when hormones begin to rage and the previously nice and sweet cockerels start fighting each other and assaulting the pullets and maybe even attacking you and/or your mother.
I had another flock in the past and I had the worst rooster ever. He never attacked me, but he used to keep my mother hostage in her own house; as soon as she got out to the yard, he attacked her. We never knew why he hated her so much.
Then, that rooster had babies with one of the hens and one of those babies turned out to be a male. At that time, I never thought about any other possibility than rehoming one of the roosters, because the first one was very aggressive and he wasn't even merciful to his own son. But at that time, my grandmother was alive, and although she didn't want to keep the aggressive rooster for safety reasons, she accepted to keep the son.
Later, the father killed one of the hens. That's why a while ago, when I realized I had a cockerel, I nearly panicked, thinking there would be a murder again. But people here told me that I had an exceptionally aggressive rooster and that not all roosters are like that.
My point is... I know they change when they grow up. But we can say that, as unpredictable as they are, no rooster can be worse than my mixed breed, bad genetics Isaac Newton (that was his name

).
I still want to do what I can to avoid problems.