The DS Dual Cartridge Adapter: Playing Two GBA Games at Once

The device you're describing is the DS Dual Cartridge Adapter (sometimes sold as "2-in-1 GBA Cartridge Adapter" or "Multi-Cart Adapter").

How It Works

Form Factor: Looks like a standard Game Boy Advance cartridge.
Primary Slot: The adapter's own PCB contains the menu/selector logic.
Secondary Slot: A female GBA cartridge slot on the bottom/underside of the adapter accepts a second GBA game.
Switching: A physical toggle or button on the adapter cycles between the two loaded ROMs. The DS/DS Lite reads whichever is active.

Console	Works?	Notes
Nintendo DS (Phat)	✅ Yes	Full GBA slot support.
Nintendo DS Lite	⚠️ Partial	GBA slot present, but adapter may protrude; some models fit poorly.
Nintendo DSi / DSi XL / 3DS Family	❌ No	No GBA cartridge slot; cannot run GBA natively.

Legitimacy: These adapters were unlicensed third-party accessories. Nintendo never produced one.
Piracy Risk: Many were bundled with flashcarts or pre-loaded multi-ROMs. Using them with your own legitimate GBA cartridges is a gray area (format-shifting for personal use); using them with downloaded ROMs is copyright infringement.
Save Data: Each game's save resides on its own cartridge—switching preserves both independently.
Reliability: Cheap clones suffer from poor contact plating, leading to "Game Pak not recognized" errors. Clean contacts with IPA if issues arise.

EZ-Flash Omega / Omega Definitive Edition: Modern GBA flashcart with microSD, RTC, save states, and near-100% compatibility. Legal for homebrew/backups of owned games.
Open_AGB_Firm (3DS): Native GBA VC injection on hacked 3DS—best accuracy, sleep mode, save states.
Analogue Pocket: FPGA-based handheld playing original GBA carts via adapter, plus many other systems.

The Dual Cartridge Adapter is a curious relic of the DS era. For daily GBA play today, a quality flashcart or FPGA device is far more reliable.

Nintendo DS