, I have a sailfin, regal, naso, p. brown, p. blue, and an achilles tang, all in separate tanks. The achilles has been the most difficult of the bunch. He is the fastest, most tempermental fish I have, and, is the biggest ich magnet I've ever seen.
Successive hypo treatments at 1.0075 for six and seven weeks at a time, (the complete fish only tank, including it's tankmates of course) still didn't cure him of it. I finally had to copper the inhabitants in a hospital tank and leave the tank fishless for two months to beat the problem.
Probably the only thing he comes in second with respect to my other tangs is aggressiveness, where the p. blue tops the list hands down.
I'm not going to tell you not to put more than one tang together, as that's your decision, but I will say that I wouldn't do it even in a 90g tank.