If you both have Steam accounts on separate computers with pure Windows setup, the overlay should work. If the overlay does not launch automatically, it means your network is not allowed, your Steam does not allow it, or your antivirus does not allow it. If the game and steam are launched with different permission settings (run as admin) then it will not work either because permissions need to match Steam. This would not be an issue for a Steam demo as it auto matches it but through Itch it has some quirks. The game is made on Windows 11 64-bit, I can not test Windows 10 compatibility. How does it run in single player?