I actually have a few in the tank that are the same genus:
Pomocanthus - Emperor and Blueface
Holocanthus - Queen and Clarion
Apolemichthys - Bandit and Goldflake
It is truely luck of the draw with the fish. In order to add more than one you need to know a couple of things about your angels:
1. How healthy are they? If it is straight out of the warehouse or just caught from the ocean, chances are they are in need of time to go into a space where they are alone and getting used to captivity and are adapted to captive eating, life, etc.
2. How big is the angel? If the angel comes in over 6" there is a better than average chance that the fish has been in the ocean fending for itself and looking for food therefore its aggression is going to be very high towards other fish. All my angels have come in under 5" and most times are around 3".
3. What is their coloration? I have found that similar color angels tend to not enjoy each other company (Apolemichthys is the only species that I have seen which is very adapting. When I had my pair of bandits, the second I dropped the new on in the tank, the original bandit immediately recognized the fish and they swam together contantly till the day it passed.)
4. How well does the fish eat? If the fish is a picky eater, it is not a good fit for additional angels. They are not aggressive fish (usually) just veratious eaters and are very territorial about food. If you have one that will eat anything and is aggressive when it comes to feeding time, you should be ok.
All in all, I know each one of my angels like the back of my hand and I observe them constantly. I know what they like and know how they will react towards others.
I also QT every fish. (minus the second bandit...which caused me to get an ich outbreak as well as flukes). Dumb on my part because if he had gone to QT, he would still be with me.
In any case, you need to be ready on the spot to take any and all fish out if they are not getting along and if one is not eating, you need to separate them in the DT or place them in a QT where they are not harrassed.