You can't make a 2 carts game.. a game cart doesn't work like a CD.
Sure you could, there just isn't any reason to. There are at least a few homebrew apps that let you remove the flashcart and insert a DS game cart to transfer the save file or dump the rom, so there's no reason they would be unable to let you swap carts.
Edit:QUOTE(CockroachMan @ May 6 2008, 06:36 AM) The save information of a game is kept on it's own cart, you can't transfer it to other carts unless you use something like a swaping GBA cart or something.. which would be stupid. On the PSX and other consoles, the Memory card is used for the that.