Just today I saw a tank with 3 unrelated males swimming around inside. Granted, this tank was HUGE and was so heavily panted that I couldn't see the bottom of the tank, but I was told that the males would just avoid each other and hide in the plants. I even saw two swim right up to each other. However, this was the tank of a professional fishkeeper, so I don't think this is a recommended practice.
On the other hand, I have a friend who kept two brothers in a large 10ish tank, just the two of them, so it is possible.
Garnet - Male Doubletail
Epsilon - Male Veiltail
Poseidon - Male Veiltail
Indigo - Male Crowntail