2 yellow tangs in a 90g?

[marie]

If your yellow tang has been in your tank for a while, there is no way he will accept another yellow tang in his tank

[Scavenger]

In my opinion, this might be possible but not guaranteed.

You would have to remove your current tang into a QT or similar tank, then completely rearrange your aquascaping, then in 2 weeks introduce both at the same time. Not guaranteed, but you might have success. The size and temperament of each tang is also a variable. In the end, it's your call. If it was my decision, I'd pass on it, the odds are just too long for comfort.