Most Butterflyfish use their sharp dorsal or top fin to defend themselves. Did you introduce these fish at the same time or did you just add the Copperband. Sounds to me like your Heniochus butterfly is defending his territory.
There's a few things that could be wrong here.
1. Your tank is to small to house the 2 of them.
2. You didn't put them in at the same time.
What can happen.
1. They might stop fighting.
2. They'll fight until the other one is dead.
3. They both die from being stressed.