You can't.
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No, you can't. Nintendo DS don't support multiplayer for GBA games.
yes
You can trade between two DS but only with DS games, otherwise you'll have to migrate them but then you cant put them back on a GBA game anymore.
You sure can.All you need is an Emulator, such as No$GBA, and the game's ROM, which you'll have to buy a card reader to get.There may or may not be websites where you can download ROMs free, you'll have to look around.
Sure you can, if you have the original Nintendo DS or the Nintendo DS Lite you can play the GBA version of Pokémon Sapphire. Otherwise there are third party DS cartridges you buy to play Sapphire.