I have a two eight month old cockerels (one RIR, one NH) that I bought at the same time as chicks and they've never been aggressive towards each other. They've lived in the same runs and coop with ten females from around 7 weeks old to present.
Four of the females are 3 year old non-laying hens that I kept just for the purpose of keeping the cockerels in line. The other six are pullets, about 9 months old.
They don't overmate the girls, though one (the largest pullet) has been the favorite of both cockerels. They don't mind sharing all the hens and I've never noticed any real "jealousy" between them.
The RIR seems to be the dominant cockerel. He acts protective, tidbits, encourages the pullets to get into the nests to lay, and I've seen him initiate the droop wing circling thing, but he just hops a step or two with the outer wing down. The NH cockerel stays more in the background and appears to understand that the RIR is boss, even though the RIR is the smaller of the two.
I'm wondering if the day will come when the two have a knock down drag out fight, or if they'll continue to get along living together with the hens.