Should I try to return my cardinal?

I had one Bangaii cardinal, and wanted to get him a friend. My LFS has had several of them in a tank for about a month, and they have always looked healthy. Well, I bought two.

One has bonded well with my larger original fish, while the other just hides in the corner and won't eat. I have seen no aggression toward him, and he looks healthy.

I have since read that 2 is the max you should have per tank. I have a 180gal. Is my third-wheel doomed? Should I just try to return him tomorrow? :cry:

Thanks.

Only a few days...