I wanted to make a post about this as I couldn't find much when I came up against the problem of two 4 week old frizzle pekin bantams (cochin bantams) trying to kill each other. They had wounds on their faces and I'm just glad I was home at the time to remove one before things got even nastier. It was seriously bad! And their mother wasn't doing a thing about telling them to cut it out.
I did find the suggestion of removing one and leaving it out until it got dark, then putting it back under the Mum to keep warm at night. I tried that and as soon as I put the one I'd removed back in, despite it being dark, they just started fighting again.
So this was my solution. I'd read an article about disciplining a rooster - holding it and tapping it on the head as though it were being pecked by a more dominant rooster. I would hold whichever chick started the fight (thankfully they are still small enough to hold with one hand) and tap it on the head with my index finger a few times until they looked submissive. It took about 10 minutes of me sitting there, grabbing whichever one thought they'd have another go, until peace was restored.
It's been a day since they were fighting and it's once again a very peaceful group of chicks. Hope that helps someone else who comes across this problem.