There are two main ways to get this working: using a dedicated or utilizing Parsec for a virtual "couch co-op" experience. 1. The "Online-Fix" Method (via Steam)
If the online fix is too complex, Parsec is a highly reliable alternative.
Only the host needs to have the game installed. Troubleshooting Common Issues it takes two online coop fitgirl repack repack
For many online fixes, using an Xbox-style controller is often required for the game to recognize the second player correctly.
Both players should have Steam running in the background. One player hosts the game, and the other can be invited through the Steam overlay. There are two main ways to get this
One player can host the full repack, while the second player downloads the free Friend's Pass from Steam. The host then invites the Friend's Pass user. 2. The Parsec Method (Virtual Couch)
The host opens the game and starts a "Public" or "Private" session in Parsec. The guest joins via a link and gains control of the second character. Only the host needs to have the game installed
Copy the contents of the Online-Fix (or similar) folder and paste them into the main game folder, overwriting existing files.