It is possible to play up to 4 players on the SNES emulator on Recalbox.
If you want to enjoy multiplayer on SNES, you will need one controller per player, properly configured.
Hotkey + B.Back twice and go to Settings > Configuration.
Save configuration on exit setting.
Return twice and go to Main Menu > Quick Menu > Controls > Port 2 Controls.
Device Type option, scroll down until you have the text Multitap displayed.
Go back once and go to Manage Remap Files. Here, select for which case you would like to save the multitap option (for the game, for the content directory, etc.).
Go back twice and select Restart.
And that's it. The game works, and you are the king of the party!