It's mostly about emulator if they implement local multiplayer.

There's another thing that might be relevant: The Game Mode UI on the Steam Deck doesn't support creation of ad-hoc wireless networks (afaik). I think (but never tried it) that it's possible in Desktop Mode though.

You can create ad-hoc wireless networks in desktop mode, which should enable you to achieve that. Then, if your emulator supports netplay, with the two SDs connected to each other, you should be able to play by connecting to localhost

That's in theory, in practice, we tried with a friend once, and his SD would just not connect to the ad-hoc network I had setup on mine

Was just looking into if I could do local multiplayer on a couple of Decks for Yuzu and it is definitely possible.

Will update how that went after the weekend.

Short version: probably not. Most emulators don't support networked multiplayer.

If the game natively supports LAN play on PC, like Minecraft or Among Us, then yes, it's possible to connect a Steam Deck to other machines via shared wifi or an ad-hoc network.

However, if the game was designed for a custom multiplayer protocol, like a GBA or 3DS game, then you're out of luck. There are rare exceptions; some games are so popular that they have mods for network play, such as Project M for Super Smash Bros Brawl. But outside of those rare exceptions you cannot play emulated multiplayer over a network.