What size are these fish? If they are smaller ones, this is a natural occurrence to show dominence and you'll need to let them either work it out or you will have to separate them. IF these are larger fish, this may be the first signs of pairing off. If this is the case, you would be wise to separate the pair into their own tanks as spawning Angelfish become extremely territorial and this fighting will continue to cause havoc in the tank.
Hope this helps