Years ago i had 2 large featherfins (about 8" each) that used to fight ALL the time. The slightly larger one always chased the smaller out of his territory,but the smaller one always came back. To me it was very strange that they would do this because i was also told they wouldn't fight. Anyway, i tried every method i could think of to stop the fighting but to no avail. The only time they didn't fight was when i was doing a water change, they would actually snuggle up together. One day i spent like 2 hours just watching the big one go at it and i noticed they it would chase the smaller one but never, ever bite it. There were never any marks on either fish, and they both ate a ton of food day in and day out. Neither of the fish showed any signs of stress and they lived together for 6 years like that until i had to break the tank down. To me one of the fish was just a little bully, claiming his territory but never doing any bodily harm. If i were you i would just let it ride and give both of them plenty of space to hide out. Observe them for a while and see what happens. Maybe one of them just needs to be the boss...