When playing the Co-Op mode of Portal 2 locally in splitscreen, there are some differences compared to the online mode. In fact, they're not differences, but rather disadvantages.
The first thing that strikes you when you start the splitscreen mode is that you're immediately thrown into the hub and all the courses are unlocked from the beginning. Furthermore, you can't unlock achievements and the gestures are not saved when you close the game.

In my opinion these problems take away the co-op experience, so I tried to find a fix for them. I looked online but some guides are outdatet and no guides have all the solutions in one place.
This guide should be a step-by-step tutorial on how to play the Portal 2 splitscreen co-op in 2024.

I have not found a solution for all of the aforementioned problems.

• you're immediately thrown into the hub
  
• can't unlock achievements (but not for all of them)
  
• gestures are not saved

• all the courses are unlocked from the beginning
  
• some gestures aren't unlocked
  
• some achievements still cannot be unlocked, if you need to play together with another Steam account (e. g. "Professor Portal" or "Friends List With Benefits")

If you're reading this and you found a new solution for a problem or improvements to my guide, let me know in the comments!

If you need more detailed instructions with explanations, you can find them in the next section.

1. Enable the "demo_viewer" beta version of Portal 2
   
2. Enable the developer console

1. Click on "Play Cooperative Game"
   
2. Click on "Standard Co-Op"
   
3. Open the developer console
   (if it doesn't open, try clicking "Back" and right after that, open the developer console)
   
4. If you want to go to the calibration course (like playing it for the first time), type into the console:
   ss_map mp_coop_start
   If you want to go straight to the hub, type:
   
   ss_map mp_coop_lobby_3

That's it. Now you can just play the test chambers. Sadly I couldn't find a way to reset the level progress in the hub, so you have to remember which levels you already played and which not.

The following section contains more detailed instructions and explanations.

First I tried this really helpful guide: https://steamproxy.net/sharedfiles/filedetails/?id=679333142 But both controllers moved both players and I couldn't find a way to avoid that. Valve probably released an update in the last 2-3 years that fixed something else, but broke this workaround. Because of this you have to play (for now) on an older version of Portal 2.
Luckily there is the "demo_viewer" beta which is a build from around 2021 that doesn't have this update.

1. Click the settings icon in your library.
   
   
2. Click "Properties".
   
   
3. Click "Betas".
   
   
4. Click the dropdown menu "Beta Participation" and choose the "demo_viewer" beta.

⠀

1. Click "Options" in the Portal 2 main menu.
   
2. Click "Keyboard & Mouse".
   
3. Set "Allow Developer Console" to "Enabled".

You can now open the developer console with "~", if you have the US keyboard layout. If not, or you want a different key, you can change it in the "Edit Keys/Buttons" menu in the "Keyboard & Mouse" settings.

1. From the main menu go to "Play Cooperative Game"
   
2. Click "Standart Co-Op"
   Now you should see the invite friends dialog:
   
   
3. Open the developer console
   (if it doesn't open, try clicking "Back" and right after that, open the developer console)
   
4. Now, if you want to start from the beginning (view the cutscene, play the calibration course), type this into the console and press enter key:
   ss_map mp_coop_start
   If you've already startet a game and want to continue (go to the hub), type this into the console and press enter key:
   
   ss_map mp_coop_lobby_3

That's it. Now you can just play the test chambers. Sadly I couldn't find a way to reset the level progress in the hub, so you have to remember which levels you already played and which not.

Thanks to the authors of these guides: https://steamproxy.net/sharedfiles/filedetails/?id=679333142 https://steamproxy.net/sharedfiles/filedetails/?id=239373369 Without them I probably wouldn't have found a solution. Check them out if you want more in depth infos about the offline co-op, or how to play it with one keyboard and one controller.

If you have any questions/feedback about this guide or related to Portal 2 co-op, feel free to ask in the comments.

Have fun!