The hen/rooster ratio is important, how much room they have also enters into the equation, but so does spirit of the roosters. Normally, if the flock is large enough, roosters will determine a pecking order. One will be boss and the other will become his able assistant and both will mate with the hens. Top rooster has his favorites and the assistant defers, but he gets his share. Some breeds, like the Olde English Game, have a lot of spirit and normally will not tolerate another rooster.
Occasionally, you will get two roosters that are evenly matched and will fight to the death to be top man. I'm talking about matched in spirit, not necessarily size. Or you will find two that are so unevenly matched, that the stronger senses the weakness and will kill the weaker, as if top rooster does not want those weak genes in his flock.
Something I saw on this site, if you have two roosters, have two different feeders and waterers. It cuts down on conflict if they each have their own.
Another option is to provide separate housing and alternate which rooster is with the hens.