You don't want to put 2 of the same types of tangs in the tank. There are 4 types that I have found info on; Acanthurus, Zebrasoma, Ctenochaetus, Nsao. Look at their scientific names and put one of each type and you should not have too much trouble. You might also put them all in at the same time so they can find their own territories and not battle for space.
We are getting a Hippo, Yellow and Red Sea Sailfin. The yellow and sailfin are zebrasomas but their coloring is nowhere near alike so we are hoping that the yellow doesn't become too aggressive.
Try to keep the type, color and shape different among your tangs and you should have some success. Remember to look at how large they get as adults. Enjoy!