It depends. If the roosters were raised together, it helps. If there are no hens around, it helps. Some breeds are much worse than others. The closer to a game breed, the worse it is. If one rooster is already with a flock and you introduce a new rooster, it could get bad in a hurry.
It depends on the personalities of the individual roosters. If one rooster is clearly more dominant than another, it is possible they will settle into a partnership and do a good job working together protecting the flock. If one is a lot more dominant than the other, he will probably kill the weaker one. If the two are matched in spirit, they may well fight to the death to be number one.
Many people have two or more troosters with a flock. It can work out quite well or it can get very bloody. No guarantees either way.