i know this is a no no but i have 5 tangs in my 92 and they all get along fine. hippo, kole, sailfin, yellow, and powder blue tangs.

it all depends
 

There is a lot of factors to consider to mitigate. IE age, aggressiveness, size, and timing of the introduction to each other. There will always be people who can do it successfully and just as many who have not. I was always in the opinion that purple tangs are much more aggressive than yellows but I have seen many subdued species and many aggressive ones as well. I am ashamed to admit but I am one who have not succeeded several times in the past because my purple was very cunning and melicious. I have since been successful for the past 6 months with a highly aggressive yellow tang that will tell the purple to pound sand with attitude. (btw for the tang polices looking to flame me, go pound sand too because I'm posting my experience in hopes to benefit others)
I believe the older and larger either species are, the more problems you will have. But if both are introduced when small (2 inches or less), neither are mature yet and I believe will learn to co-habit with each other (i believe many species of juvenile tangs school in the wild until they get a little older for better chance of survival). That is (IME) one of the better methods to get them to be in the same tank together. When they start getting bigger, they are more unpredictable and lessor chance of success of cohabitation.
The cons to that method is that the smaller tangs' survival rate is not as good when smaller.
Hope that helps