I own the game Elite Beat Agents, and I have been playing versus mode with a friend with DS download since he does not have it. We can only play around half a dozen songs on breezin' and cruisin' difficulty.

Are more songs and difficulties playable if he gets a cartridge of his own?

well, wat do you expect? it says right on the menu you pick it on that there is a LIMITED version the other person gets if they dont have the card. wich should mean that if you and the other both have cards im PRETTY SURE you both get all difi. all songs and any characters you unlocked!