Have Player 1 select the desired character and costume and press A. Have player 2 make his cursor hover over the desired charcter (don't press A yet.) You have to have the P2 costume as one of the ones that you only have to press X or Y once to get to the desired one. For example, if you want two Links with green costumes, have P1 select that with A and have P2 hover over Link with either the red or white costume. Deselect P1's character and change P2's costume AT THE SAME TIME. If done correctly, both players should have the same charcter with the same costume.
Submitted by: Agent Retera