They were almost certainly fighting over hens. Changes in flock dynamics can cause tension and fighting.
Was the person you got them from lying? Not necessarily, they indeed might gave historically gotten along but that changed when they got introduced to their new flock. In any case, how they were together at their old home is irrelevant. They are fighting now. Whether they will continue to fight in the future remains to be seen but it's a very real possibility they will and the only real solution in that case is permanent separation. That can be splitting the flock, having one in a separate pen, rehoming or eating one of them. Doesn't matter what you choose, if they continue to fight they just need to be separated permanently.
If they appear to get along, still be cautious as this may or may not happen again. I would think long and hard about why you want 2 roosters and consider if you have a way to separate them in an emergency.
Click here for tips on safe separation. That needs to be in place long before anything else happens.