I thought that if two people wanted to play a DS game, only 1 person would need to have a cartrigde while the other downloads it. But thats not true. I have Mario 64 DS, Madden, and the Metroid demo, and only mario 64 uses the 1 cartridge download system (i don't know about any other games). You need 2 cartridges for the other 2 games. My brother brought home madden the other day and i was so excited that we can play together via the DS download, but i read that u needed 2 cartridges for multiplayer and was pissed.