You have to 2 memory cards, which decks already configured, then select 2 player mode, pick your stage or if you have a premade stage you can use that, selet whether or not you want the players cards to be showing and then your good to do
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To play two player in Yu-Gi-Oh Duelist Of The Roses, have 2 save games and put both memory cards in the ps2. Then go to "Custom Duel".
The rules are here: http://entertainment.upperdeck.com/yugioh/en/gameplay/rulebook/rulebook_v06_EN.pdf
You could play it on both. Any DS game can play on your DSi
In a 'casual' game amongst friends, there is no real reason why you can't play by whatever rules you want. There is nothing stopping you using a fake card, if the other players agree to it.