To launch multi-cart mode :
Go to the main menu, select any option, press Select (you should see "Debug Menu MPDS" appear), then press OK.
For the host console, select the "MultiBoot Parent" option. For the client console(s), select the "MultiBoot Client" option, then the host should appear.
Press Start once all clients have joined. You'll return to the main Debug Menu.
To launch a board, use the "Board Entrance" shortcut. You can press "B" in party mode to access all other game modes.
More details on the Debug Menu at: https://tcrf.net/Mario_Party_DS#Debug_Menu

/!: With Multi-Card, client game saves will be replaced by the host game save!
Hack link :
https://emultrad.fr/hacks/mario-party-ds-multijoueur-multi-carte-mode-debug-hack-nds/
https://romhacks.org/romhacks/mario-party-ds-multi-cart-multiplayer-debug-menu-nintendo-ds/