Generally I try to keep tangs that do not look alike in shape and size (except yellows) as that will minimize aggression i.e. Powder blue and Powder brown are to similar and can have aggression. This isn't a definite as size of tank, rock scape, feeding habits etc plays a large factor. If you don't have any yet it's best to add them in at the same time if possible so they dont have claimed territories. Feed lots of Nori to keep down aggression. If there are tangs in already you can hang a mirror near where they hang out to distract the original one or use a fish trap to get them adjusted to seeing another tang in the tank. In my tank right now I have a Blond Naso, Dussumieri, Desjardini and a yellow belly regal, no real aggression from any of of them.
