I don't know abouut single card, but I highly recommend you and your friend go out and find two copies of Glory Days 2. You can play co op and vs with up to 6 AI aircraft, split however you like- 2players vs 6AI, 2P+2AI vs 4AI, 1P+3AI vs 1P+3AI, or any other combination you can think of between 2 and 8 aircraft total. Hands down the best multiplayer DS game of all time. Contra 4 is the only other game that comes close.
I wouldnt mind knowing the list of 3DS games that are single card download and play. My roomate might get a 3DS and doubt he is the type to buy alot of games for it, so it would be nice to know what we could play together with only one card.