One yellow tang or a large group (school) is the responsible answer, assuming someone has room for a school.
Yellow tangs are mean, often a pair becomes one and often groups become one but there are no absolutes. There is always going to be someone out there who has a pair together with no problems. Often it is these same people who advise that is is a good idea to have a pair of them not knowing that they are the exception.
Also, two small tangs may become two big tangs...then what?
My advise on tangs is to get species that are completely different from one another in color, shape and habits. (For example, a yellow, blue hep and a naso sp.)They are less likely to quarrel with one another.
Joel