I have a hippo and a yellow in a 180. They chase each other occasionally, but both have been in the tank and healthy for almost a year. I've never seen any real aggression. With most tangs, as long as you don't have two that are the same color and body shape, aggression won't be a problem. Two yellows, two purples, two hippos, ect... can be a problem. You can, with a bit of luck, keep them in threes if you have the room. It isn't recommended though.
Life is not measured by the number of breaths we take, but by the moments that take our breath away.