Multiplayer Error: "Joining failed - You're already connected"

This error occurs because your player manifest in appdata contains the other person's UUID, which can happen if you are sharing a PC, account, or saves.

Wiping the folder is the easiest way to remedy this.

On each computer:

- Disable cloud saves
- Back up saves in c:\users\you\appdata\local\larian studios\BG3... 
- Delete the entire BG3 folder
- Launch the game again to regenerate the folder automatically. 
- Put back any save files that you wish to keep. 
- Play the game

To disable your cloud saves: 

You can disable Steam cloud support either globally or for only this game. For disabling Steam Cloud globally, within the client click on the Steam menu and select Settings, and then Cloud. Or just for this game: go to your Steam library and right click BG3 and select Properties, then switch to the General tab and check the Steam Cloud section.