It appears that your Angels are fighting for territory. It's pretty common thing for them to do once place into a tank. They get even more aggressive if the tank is too small. Some fish enthusiast recommend at least a 40 gallon or more for two angelfish.
Can you tell me how long they have been fighting and how big the tank is? Sometimes when they are first introduced, they will definitely fight for territory, especially between the same sex. Once, one of them knows who is the boss they can tame down a bit. But if you feel that their fighting is almost causing the other fish to not eat or if his/her fins are badly ripped than you would have to separate them. Either return one or get a bigger tank. Might want to have some tall plants or driftwood to give them some hiding places, especially the smaller one.
Or if you had them for a long time, they might have paired up and started to mate? I doubt it if you said that one is practically destroying the others fin.