I agree with Bob and yet I've had success with putting two like Tangs together in groups or simple pairs.

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