Try to pick one that is entirely different color and/or shape wise. If you can't then at least make sure the new one is the same size or bigger than the established Tang My PBT went after my Kole like a banshee when I first introduced him even without a similar color or shape (though only half the size). An hour of beating himself up in the mirror solved that, now they are best friends.
I mention that because I have heard Yellow Tangs can be aggressive to their own and similar kind.
65gal drilled Reef (3'x1.5'x2').
1 Powder Blue Tang, 1 Yellow Eyed Kole Tang, 2 Ocellaris Clownfish, 1 Talbot Damsel, 1 YWG, 1 Red Honey Damsel.