They are living animals so anything can happen, good or bad. Part of it is breed dependent but a lot depends on the personalities of the individual roosters.
Adding a rooster to a flock with an existing rooster is extremely risky and I'd predict bad things are likely to happen. It can work out, but is risky. If two roosters are raised together, the odds improve quite a bit. If they are raised together, they may work out the dominance positions while young and form a good partnership in taking care of the flock.
When roosters strive for flock leadership, three things can happen. If one establishes dominance over the other, then they can coexist. The dominant one will have his pick of the hens but the assistant gets his share. Often this looks like you have two flocks as they don't necessarily hang out together. If you have limited space and/or a small number of hens, the chance of conflict rises, but two or more roosters are successfully coexisting within with many flocks right now. If the two roosters are pretty evenly matched in spirit, then neither will accept the assistants role and they fight to the death. Or if there is too great a difference in spirit, the stronger will kill the weaker.
You will see recommended ratios of 12 to 15 hens per bantam rooster. It is 10 to 12 for full sized fowl. This is based on fertility. One active vigorous, reasonably young bantam can probably keep 12 to 15 hens fertile. While there is a very definite relationship, this ratio is not based on barebacked hens.
When a rooster mates with a hen, he can remove a few feathers (not unusual and not all that bad), he can remove enough feathers that skin is exposed (getting serious and requires action) or he can injure her, usually with his spurs (often fatal as chickens are cannibals). Sometimes this is caused by a rough technique, long sharp spurs, or the rooster is fairly heavy, especially in relationship to the hen. But often it is caused by overmating. Sometimes a rooster will pick favorites and overmate with them; sometimes there are just not enough hens to handle his technique and vigor. The problem can be worse if you have two or more roosters. There is no magic hen to rooster ratio that guarantees bareback hens will not be a problem. Some people keep one rooster with two hens and don't have this problem. There is an active thread where the poster has 18 hens with one rooster and has bareback hens. I don't know for a fact that specific problem is overmating with his favorites, but I think it is likely. A ratio of 8 hens and 2 roosters does not guarantee that you will have this problem, but it is something I'd advise you watch for.
