Nintendo DS + DSi Multiplayer: Yes, Fully Compatible—Local Wireless & Download Play Work

Yes. Nintendo DSi plays all Nintendo DS games and connects wirelessly to DS, DS Lite, and other DSi/3DS systems for local multiplayer.

How It Works

Backward Compatibility: DSi runs DS cartridges natively (same ARM9/ARM7, same wireless protocol).
Local Wireless (Ad-Hoc): 802.11b/g — range ~30 ft. No router/internet needed.
Download Play: One cartridge hosts; others join via "DS Download Play" (single-cart multiplayer).
Nintendo Wi-Fi Connection (Defunct): Official online shut down 2014. Replaced by Wiimmfi (fan-run) for supported titles (Mario Kart DS, Pokémon, etc.) — requires homebrew DNS patch.

DSi-enhanced games (black cart, TWL prefix) run in DS mode on DS Lite/Phat — multiplayer works.
DSi-exclusive games (white cart, DSiWare) do not run on DS Lite.
3DS/2DS also compatible in DS mode (screens stretched unless "Native" held at boot).

Grab a friend, two DS systems, one cart (if Download Play supported), and play.

Nintendo DS