x superbox

You add them to your account, like you would do with every win on this site, unless you want the ban hammer.

I would recommend that you thoroughly read the FAQ if you want to stick around.

If you mean you were expecting only 1 key and want to give the "spare" back, it's more likely that the 2 keys come from the Humble Weekly Bundle, 1 key gives you X:BTF, X-Tension, X2 and X3:R; the other gives you X3:TC and X3:AP and makes it equivalent to the Superbox.