The tank size makes it worse.
Tangs generally compete and you have to be careful of which tang you pair with which other tang. That combination would likely work if you had a much larger tank. At least 100 gal and 6ft long. You could maybe keep the yellow if you got a 5 foot tank but for two I would add the extra area,
Please do not add another tang to this tank and if possible re-home the yellow tang. Then you can get maybe a really pretty wrasse. Many are colorful and quite fun. Yellow Corris wrasse would replace the splash of yellow from the tang.