I gotta agree..and have experience against the "two" tangs thing. I had a blue hippo (5") and an Achilees (5") that got along together about 3yrs. They never had 1 incident - pretty much just kept to themselves. The blue hippo popped in and out of the rock, while the Achilees stayed out in the open swiming back and forth like he was on crack.
What i've heard about the tangs, more than some sequence of numbers, is keep them of different body shapes. And if one is more agressive than the other (ex, yellows tend to be a more aggressive tang than the much dosile hippo) - it's best (if possible) to put the MOST DOSILE one in first to give him time to estabish a territory and comfort level AND IF POSSIBLE, make the more dosile one LARGER in size.
I've had 1 tang, added another (2) and had no probs. I've seen others with 3 VERY often. Just keep in mind, they will eat/need similar food sources. Don't make them have to fight for a low supply. Keep the peace
Good luck to ya. Aren't Tangs great??