This one is very new, and I haven't seen it mentioned much on any platforms. The developer (Kyle McParland) has developed an app for Android (and iOS / PC in the future) which lets you and a friend 'pass the controller', and play any of your emulated ROMs together, with co-op.
It's called Your Turn!
He's very much in the early stages of the app now:
How it works: you and your friend load the same ROM on the same emulator on your devices (currently supported emulators for BETA: John GBA or John GBC - lite or otherwise). Open the Your Turn! app and invite your friend to start a game. Take your turn, save the game using the emulator’s save state, add a note, and upload it in the app. Your friend gets a notification, reads your note, downloads your save, and plays their turn. Repeat back and forth - just like passing the controller!
To me its nice, because it encourages a kind of game-play we've left behind in the modern gaming scene. I also love that it is a unique take on playing through much older games, which before this weren't 'sharable'
Only some regions are supported right now, but more are being added constantly as the testing is getting underway:
What I love is that he hired an artist to create the official art/instructions for the app, and he now has a mascot for Your Turn!
That artist is Evan Palmer, he can be found on Instagram @evantickles
Anyway, I know there's a lot of gamers in here who enjoy the older games. Either from a collecting standpoint, or from an emulating one. I wrote a feature article on the app after chatting to Kyle a ton, so if you want to read all about it, you can with the following link
(...I love that mascot, so much)
I'll ask the dev, and let him know. I agree with you, I loathe Discord (I only re-joined recently in order to reach out to and interview devs), but from what I know so far things are quite early in the app's world.
I'll let him know you're looking to try, and send him a link to your comment here :)