Depends on the size of the tank most of all.
Assuming you're at least 125g, you'd be OK with 2 in there. I've had 3 in a 155g with no problem - prolly 'cause they were from 3 different genus (is that the right word??). I did the Purple, naso, and a bristletooth. Tangs don't normally like those that have the same body shape/look like them. I was lucky to have two tanks set up (neither of which where the 155g) with the tangs already haven gone thru QT. That way all 3 were added at the same time so no one could claim it as their home already and be the bully.
Gonna have to give some thought about the size of your tank, which tangs are going in there and the order and size of each as they go in.
Tell us which you are thinking about and we can prolly give you better advice. Oh yeah - -what sized tank we talkin' anyway? Could be a mute point.