Wil: That's how it plays on this site. There's been an ongoing debate about it too. You either make whatever move you decide or you make the move as allowed by the rules that you and your opponent agree to play by. Since you called it an illegal move, I'll assume that you're used to playing that if only one die can be used the higher must be used, and that you also play that if both dice can be used then you must use them both and a player isn't allowed to play one die in such a way that the other die cannot be played if it is possible to play both dice by making a different move.