If there is no apparent damage happening to either fish I would let them be, as with a tank that size there is enough room for one or the other to escape to. If you start noticing damage to either, a divider would be the best starting place if you wish to keep them as a breeding pair.
Breeding pairs of angels generally are not show specimens when actively breeding, some torn fins & loose scales often happen. Lip locking & pecking are pretty normal, as long as it doesn't get to the point of a stressful situation or serious damage. Angels are cichlids, and anyone keeping cichlids is wise to keep a divider or extra tank handy.
Separating the pair & rearranging the tank, letting the more docile fish settle in the tank, and then adding the more aggressive fish is fine for territorial aggression. This is not the case in your situation, you have angels showing pairing aggression, which is usually more nasty looking than it actually is. Angels get really torn up fast when stuck in a situation in which there is territorial aggression, this is where relocating & rearranging works well.
At times, even the most well mannered of pairs can have a falling out, this can happen at any time, and for no apparent reason. Paired angels have been known to get aggressive to the point of killing a mate, this is one reason a divider is good to have, just drop it in, much easier than setting up tanks, catching & switching fish, and so on.
Breeding fish often release hormones into the water, this includes angels. By switching one of the fish out you can interrupt the breeding cycle, and this release of hormones. Replace the more aggressive fish, and you may have broken the pair. They may re-pair, or the breeding aggression may end up being territorial aggression, which generally causes much more damage to the less aggressive fish.
How do you tell? With territorial aggression the more passive fish will do everything it can to hide or escape, this means hiding behind or under objects, or going for the surface in a horizontal position. The only time you may see the fish near each other is when feeding, and the aggressive fish will chase the more passive fish out of a clearly defined territory shortly afterwards. Paired angels who like to test each other, which is common, will tend to stick near each other for the most part, with little relationship spats here & there. They will both defend a common area, especially before, during, & after spawning.
