Install the windows XCOM2 binaries by opening the XCOM2 properties before the actual install.. Go to "Compatibility" and select "Force the use of a specific Steam Play compatibility tool". You can select a different proton version if you want to.

Start the game once. You only need to let it go to the default launcher. Then exit. Just to make sure everything works so far.

After XCOM2 is installed go to the .../steamapps/common/XCOM 2 directory and create a directory there and unzip the alt launcher there

Now the alt launcher is parallel to the regular launcher.


Change Launch options (the easy way)

If you installed the AML in parallel as described above then replace the Launch Options in the properties of the game with the following:

Thanks to Xeloses for the hint. Below is the first (hard) way that I did. A bit over engineered but works as well.

Change Launch options (the hard way)

Why would you use the hard way? If you want to set certain environment variables or do additinal things, then it is easier to do that in the script we will create before

Go back to the XCOM 2 properties. In the "General" tab  change the "Launch options" to. Replace "user" with your username. If your home directory is somewhere else change it to that.
Then start the game. It will not start. Instead there will now be a file xcom2_run in your home directory that gives you the exact command that steam executes here.

Now there should be a file called xcom2_run in your home directory that looks like this. The user would be the name of your user. My steam library is installed in /opt/software/SteamLibrary/. That path should be different for you.

Now edit that file with your favorite text editor and replace 2KLauncher/LauncherPatcher.exe with the directory you created and the launcher  executable. In my example:  alt_launcher/XCOM2 Launcher.exe
Also add a #!/bin/bash in the first line. Your file should now look something like this:


Make the file executable with

Finally go back to your XCOM 2 properties and change the launch options from the echo we put there before to
The %COMMAND% is important, else steam will just add the script at the end of the command instead of "replacing" it. The script itself does nothing with the %COMMAND%.

Run the game again and it should start the Alternative Mod Launcher.

Because I had moved my steam library to a different drive, the launcher couldn't find my mods. I had to add
to the xcom2_run script. Right between the "#!/bin/bash" and the command.