My son has Yugioh: the eternal duelist soul for GBA and I would like to duel against him using my GBA. Are the Yugioh GBA games compatible with each other in 2-player mode or do both players have to have the same version in order to duel? I know there are differences with the cards available in the different versions but I thought maybe the game sort of downloads the cards from player to player as needed. Thanks for any info.

Check the back of the game's box, or manual. If it has a symbol or text stating that it supports one cart multiplayer then go ahead. But i suspect that Yu-Gi-Oh would be mainly battling player-customised decks (only know a little about Yu-Gi-Oh, so I may be getting this wrong), which would mean that there's no way the game could run on its own.

In order to duel your son on your GBA you would need a copy of the game as well, with a custom deck loaded and so on. I used to own the game and play against a friend of mine, and you each have to have the cartridge present in the system. Hope thats what you're looking for.