If they are totally free range they should be OK. Make sure there are no places one rooster can corner the other, thats when one can kill the other - as it can not escape. Also give them each their own roost place - maybe use a small rabbit hutch for one to sleep in away from the other.
I have Japanese Bantams also. I often add new birds to my flock, and I have about 7 roosters now. When a new rooster is a added all the roosters will fight with him, and a little with each other, to sort the pecking order out. Normally they will only have a quick fight lasting less than 5 mins, usually only a couple of minutes. I am always there to make sure its not getting out of hand. The looser just runs away and stays out of the way of the winner for a while.
I only had one time that was serious. This was after I sold the top rooster. He would always break up fights between the lower ranking roosters. After he went, the 'second in command' roosters when crazy and kept fighting - they fought really bad - neither would give up, even when both were exhusted, laying next to each other panting. They were not injured - just a few bits of blood on their wattles. They kept getting their breath back and going at it again.
I put one rooster in a cage away from the others. Then I went to work. When I came home they had been fighting through the wire mesh of the cage - they were covered in blood and really beat up faces - eyes closes, laying limp on the ground. One had a really bad limp and i thought his hip was dislocated. I was really upset, and I thought they would die. I could not understand how they did so much damage through the fine wire, and also why the one in the cage did not just step back out of the way!
However, they recovered fully within a week - like new. I kept one in the cage in my outhouse - and planned to sell him. But he got out again when I was at work. When I came home they had had another small fight and they were getting on fine together. This was a year ago and they have never fought again.
Birds are strange! I want to show you that they can get on in the end, but I would never want to go through that again. If yours still fight badly and one won't back down after a 10 mins, then I would re home one.
