Separate them, or the larger will likely be dead before many days/weeks have passed.
Now for the reason. Angelfish are a shoaling species, meaning they need a group; five is the minimum, but more is always better. Within this group, an hierarchy will fairly quickly form. All males are territorial, but the group allows this to develop into an hierarchy. With two, three or even four angelfish in the small (to the fish) confines of any aquarium, there is insufficient numbers for the hierarchy to work itself out. Sometimes things may work out, if all are females for example; but males (assuming they are healthy and normal) will not do this.
The more there are in the shoal, the more spread out the natural territorial nature will be, which is why groups of 12-15 in a very large tank can live together for years. In the natural habitat, a picked on fish can swim away, but that prerogative is not offered in the aquarium unless it is indeed huge.
ven a male/female may not get along. This species must select their mates, and bond, if they are to live together for any period of time. Sometimes we may luck out and put a male/female together and they do bond, but this is rare. The same principle applies to many cichlids, such as the rams and some other dwarf species.