I think you are going to see little personal experience with having multiple black tangs in the same tank due to their price tag but I don't think they would be that much different from having multiples of other zebrasoma. I've seen people with multiple gems tangs that aggressively attacked each other to the point they needed to be split up, just the same as I've seen it with yellow tangs, so there is always a chance you will need to intervene at some point (most likely when they mature).
Provided you have the space and follow the normal rules, ie add them together and try and get slightly different sizes, I don't see why it would be more of a problem than other similar temprement tangs. I have seen people add hybrid blacks to blacks with just a little roughing up.
