I can only offer limited knowledge, as I have only owned cattle for around 3 years. My father in law, however, was in the cattle business for over 40 years and always kept around 80 brood cows. He always ran 2 bulls together and in the 10 years which I spent around his cattle I never once saw a serious fight, just minor shoving matches. I have personally witnessed one of the bulls breed a cow and the other bull watch, and fifteen minutes later, the other bull breed the same cow and the first breeding bull watch, with no aggression expressed between them. He said he always bought bulls that were raised together and never had trouble with them injuring each other. I currently own 2 bulls which were raised together ( I raised them, both are 2 year old reg. Charolais), and in two years I have never witnessed a fight between them. I spend time with my cattle every day. They know which of the two of them is the boss. My father in law has always had Charolais cattle, as do I, so maybe it's just the breed. He never had a defined calving season, and I also don't separate my bulls and cows (plan to in the future), maybe this has something to do with it, I don't know. Maybe we've just been lucky.
Trey