Steam – Disable Two Factor Authentication

If you want to do this without starting Steam and logging in, you can modify the setting in the Windows registry. The autostart checkbox is directly tied to a registry key — simply deleting it is all the Steam client does when said checkbox is unchecked.

Navigate to HKCU\Software\Microsoft\Windows\CurrentVersion\Run and delete the Steam key. Or as a .reg file:

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run]
"Steam"=-

Assuming his backups were working correctly (backup after every session), it should be safe to overwrite his save with yours, then let him overwrite yours when he wants to play.

The easier and safer option would be to use separate Windows accounts, but older games tend to store the saves in their install folder instead of your Users folder, putting you back at square one.