I developed a modded emulator for Pokemon Red/Blue that allowed multiplayer (as in, multiple people walking around in the world, all interacting with the npcs and being able to battle each other) but unfortunately when I asked around there seemed to be no interest. I still have it on github, maybe some day I'll get back into it.

https://github.com/Salgat/PokeSynch

Oh but there is shared game state! Most of SRAM data is synced for ALTTP and there's even a PvP feature so you can slash at each other with swords and use some of the items to damage each other. Of course you can enable/disable whatever bits of state to sync or not.