I agree that you are better off with just one. There is always the possibility that they will fight, some territorial aggression is normal with tangs. What is a certainty, however, is that eventually things will get cramped for two, since all tangs grow to a decent size. In this case, you will be faced with which one to get rid of, and where it should go. That is not a pleasant thing to think about, and is a situation that I would say is best to avoid since you can.