Hi Twopenny
There is a alot that can be said to answer your question. I am sure others will chime in as well.
Angels can do well in pairs/multiples in the right enviroment/setup, but they are/can be aggressive fish under the right circumstances even if they appear to have a genial temprament.
Angelfish breed in pairs and the process of selecting a mate can usually be a terrifying one to watch. Angels have been known to pick off other tankmates when the "pair up", in order to secure a safe place to spawn. Angels establish hierarchy amongst themselves. So a few things are important to consider before you put a new fish in...first, do you know the sex of your current angel? Do you want to possible have angel spawn or to breed them? How big is your current tank and what other inhabitants are there?
Any fish you add will likely result in a display of dominance. If two males without a female, two females, or a male and female...all will lead to some display of what you will likely view as aggression...chasing, nipping, crowding the top of the tank, lip locking...but for different reasons. Their behaviour can be complex, but all very interesting as you get to know them. If you don't know the sex of your fish, then just be prepared for these behaviours and depending on their age, an eventual display of their sexual attributes.
If your tank is large enough to add another angel, I would recommend 40g+, you certainly can add another. First, select a fish that is the same size as the one you already have. I suggest, after a period of quarantine, adding the newest angel at night after lights out. This way the new guy can explore in a less active/potentially aggressive environment. Then watch the dynamics closely to be sure one isn't getting beat up too badly and is getting enough food.
Freshwater Fish Compatibility Chart - Rate My Fish Tank