How does dual-slot mode *technically* work?

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Is there a way to enable the use of a Game Boy Advance cartridge on Slot-2 while playing a Nintendo DS game? It's usually seen on Generation 4

ds slot DS GBA GameCube Meta Quest Nintendo 64 PS1 PS2 PS3 In the original title, you would need to assign the ocarina to an item slot

ds gameboy slot This allows the NDS slot to be held inside a mock Game Boy cartridge Any issues please comment and I'll work to fix them!