I owned a blue tang, and added a yellow tang, and Scopus tang on the same day (to avoid any conflicts) to a 75 gallon tank that I have. It didn't work out. The Scopus killed the Yellow within a couple of days. I ended up removing the scopus (which has a similar temperment to the purple tang,) days after purchasing him. Be prepared to remove one or the other. I now have three small yellow tangs, and 1 blue. The yellow's leave the blue alone completely. It is best to keep the same color/type of tang in odd numbers to prevent them from picking each other off. Even with three of them however, they still harass each other here and there.
My personal experience is that if there's any fear in your mind about compatibility, those fears will usually become a reality if you chose to do it anyway. Best not to do it, and save yourself money, and the hassle.
My 2 cents