mchandler,
Keeping two tangs, with dis-similar body shapes, in a tank as big as yours should not be a problem as long as your water parameters are good. Looks like your ph is borderline low. You should try to bring it up to the 8.2 to 8.4 range. You also should be working at getting your nitrates down to 0......water changes can help.
I'll guess that your tank is relatively new? If so, I'd wait a few months before adding a tang. Tangs do require good water conditions so allowing your tank's water chemistry to stabilize can help prevent an ich outbreak.
Also, as Kip mentioned, you want to make sure you're feeding tangs a steady diet of greens. They require greens to stay healthy.
Jim