Here's a nice compromise between the legality of using only legitimate DS cartridges and the (albeit less legal) convenience of having multiple games on the one cart: a device that lets you switch between legitimate cartridges.
This is the Blaze 3-in-1 Game Selector, and while it looks a little clunky, it makes up in handiness what it lacks in attractiveness. Working like a switchbox, the device replicates the cartridge slot of your DS and clips to the back of your handheld, allowing you to insert three DS carts at once. A small lead then runs from the device to your DS's actual cart slot, and you change games by flipping a small switch on the top of the unit.
The 3-in-1 Game Selector runs for about $US13, and if you're stuck playing DS game in a spot where it's inconvenient to keep bending down and rummaging through your bag to change a game (standing on the subway?), that's not a bad price.