To accomplish this trick, you must have two separate saved games. You can't just save to a different file or memory card. Start a new game, abort the first mission, and save your progress in a separate file.

Now go to the VS mode and load your primary game (the one you want to copy your cards to) for player 1, and your new game for player 2. Wager whatever cards you want to duplicate, then lose to player 2. Load your new game, then save that game in however many slots you want (don't overwrite your primary game!) Go back to VS mode and load your primary game as player 1, and whichever copy of your new game as player 2. Win as player 1, and take back your cards, then go through all the duplicate new games, making wagers of whatever cards you want duplicated. This even works with the very expensive capture card!