You cannot directly link two Nintendo DS consoles with a cable. Instead, multiplayer relies on wireless methods:
Many DS games support ad-hoc wireless play for 2–4 players within approximately 30 feet.
Some titles allow a second player to download a limited version from the host's cartridge, enabling single-card multiplayer.
Formerly enabled online play, but this service is no longer functional.
Certain downloadable games offer local or online multiplayer.
Key requirement: Most full-featured multiplayer requires each player to own a game cartridge. Check the specific game's manual for supported modes.