if you add them at the same time i dont see a problem, also you can add mirrors to the sides of the tank to reduce aggression, if you already have one or the other in there, if you have red leds in ur lights turn them on an 1 hr after lights go out and put the fish in the tank, fish cant see red so its pitch black for them but we can see them. when they wake up there will be minimal to no aggression between the 2.
i saw your post from before. now i will say, keep in mind that the PBT's are very tricky compared to the hippo tang and naso tang. the Naso tang is also tricky if a juvenile. Hippo tang and PBT are ick magnets, slighty more than another tang.