How to enable online couch-coop for your emulated games on Steam Deck

One of the greatest allures of gaming is sharing an experience with your friends and family. Be as online gaming shared with your buddies over voice-chat or sitting next to your family while owning them in Mario Kart.

Nowadays, a lot of games come with online multiplayer baked in. But what about your favourite retro games that don't even have servers online anymore?

As Redditor occamcs has discovered, there is a way to enable couch-coop on your Steam Deck via EmulationStation to invite your online friends to your in-game session. occamcs has successfully tested online couch-coop with Dolphin, Cemu and Yuzu, and I was able to have a session of Mario Kart 8 with them running Cemu on their Steam Deck and me connecting to their session using my Steam Deck.

The setup is relatively simple but includes a few steps. Unfortunately, currently, it only seems to run in Desktop mode. Here is what you need to do:

1. On your Steam Deck, boot into desktop mode. Click the Steam button, go down to Power, and then Switch to Desktop. Wait for the Deck to reboot.
2. Open up the browser of your choice, and go to the Github page for RemotePlayWhatever. Download the latest AppImage. Side-note: RemotePlayWhatever gets updated every time there is a new Steam Client update.
3. Go to your download location, and move the AppImage to home/deck/Applications. Next, launch the AppImage. You will get an icon in your system tray.
4. Next, you need to start the standalone emulator you want to use. occamcs has successfully tried RemotePlayWhatever with Yuzu, Cemu and Dolphin.
5. Right-click on the system tray icon for RemotePlayWhatever, and invite a friend via Steam. Their controller should automatically connect, whether an actual controller or the Steam Deck.
6. The next step is a little cumbersome but only necessary once per emulator: switch into the gamepad setup in your current emulator. This step is essential: Make sure controller 2 is set to xinput and is set to the second, newly connected controller. Depending on the emulator you're currently using, the controller's name will be different but should bind automatically. Don't worry if the connected friend's input is not registered in the standalone emulator. This is normal. Also, it will look as if you have connected two controllers in the Remote Play Together host window and that the connected friend has none.
7. Now open EmulationStation, and start the game you want to play with your friend. In our tests, we could play Mario Kart 8 via Cemu. Unfortunately, it seems that only EmulationStation can detect the remotely connected controller despite us setting it up in the standalone emulator.

That's it. You should now be able to play emulated games with your friends, as long as they have a local coop.

Some notes

• In some circumstances, it might be possible that whoever is connected to your game cannot hear any sound. The current workaround is to call them through Steam and use your audio monitor as input. This will use your game audio as the microphone, and they will be able to hear the game (though not you). To do this, you need to: Call them, then deafen yourself. Next, open up Pulse Audio Volume Control, switch to Recording, and change the drop-down to the monitor for the speakers.
• The developer also released a version of RemotePlayWhatever that is compatible with the Steam Deck's Game Mode. But while the experience of using it is better, this feature still suffers from a few bugs and runs into the same audio issues as the desktop feature. So consider it a beta. But if you still want to install it, simply follow step 2 and 3 above, and later add the installed AppImage as Non-Steam game to Steam: For this, you need to open up desktop Steam, go to Library, and at the bottom left corner, click on the plus sign, then Add a Non-Steam Game.... Navigate to your installation folder and add RemotePlayWhatever.
• There is a separate Github user currently working on integrating RemotePlayWhatever into the Decky launcher.