A lot of people do it.
I think the best thing to say is it isnt recommended. I'd never tell a person new to the hobby they should add two acanthurus or any two tangs to the same tank. However, an experienced hobbyist can probably easily get away with it, especially if you're willing to remove one tang if it doesnt work out and maybe try a different tang or at least a different size tang of the same species as was removed.
I've always had multiple tangs in my tanks, sometimes I had issues and I removed them as needed or juggled things around. I have a good relationship with my LFS so it's easy for me to move fish around.
My 210 I had a 10" dussumieri tang and a 3.5" achilles with absolutely no problems after the first week. The first week though the dussumieri would have killed the achilles if it could have. After that it just tolerated it, I never saw it chase it again though and the achilles didnt look nervous after it or flare it's fins when they were together so I figure they got things settled.