They will fight until one of them concedes. If they are both just as hard headed, then yes, one will kill the other. I don't know about cochins, but I have been surprised by the tenacity of a Japanese rooster. If you separate them, you will have to keep them separated. Every time you put them back together they will start all over. If you choose not to let them battle it out (there will be some injuries even in a quarrel) then you will have to keep separated or cull. How many hens do you have? Some times the answer is more hens or more space.