I agree with Bob and yet I've had success with putting two like Tangs together in groups or simple pairs.
Adding new fish, Fish Training and the Miracle of Garlic Oil:http://utahreefs.com/forum/forum_posts.asp?TID=34014
The Reefkeeping Tip copied above explains how two things make all the difference:
1) training - performed by the hobbyist; and
2) the environment - the perception of there being sufficient food growing in the aquarium, (supplemental feeding by the hobbyist is often insufficient). I included a story there about two Tangs, a Yellow and a Purple (both Zebrasoma species with the same body shape and close in size) that became close buddies, but it took training and patience. There were a couple weeks of watching them beat each other up with one of them
seeming to be the apparent loser.

In the end, I learned something more of fish mentality and behavior.

Years before this I had a Tomato Clownfish pair where before the pairing the female had almost ripped the jaw off of the male. He healed up and about 8 months later the two became the parents of my first baby clownfish.
My opinion is that we can learn a lot from fish in how they overcome differences, practice forgiveness, make friends and even become partners.
Aloha,
Mark

Edited by Mark Peterson - July 26 2015 at 12:41am