You must choose two pieces that each have more than one move available, so that your opponent does in fact have a choice. For example, in the opening position, suppose you choose to move the Ph2 and the Ng1; one of Black's choices is h2-h4 and Ng1-h3, but he cannot choose Ng1-h3 blocking the Ph2 (he can move either piece first, but if the N moves first it must go to f3; or if the P goes first and stops at h3, the N has no choice; but Black had a choice, so the doublemove is legal).
You will not often choose to make a doublemove in this game, but must always think about it; and that is the idea of the game.
Compare with Compromise Chess and Refusal Chess.
The fool's mate is faster than in FIDE Chess: W can mate on move 2.