Open the same project more than once on one computer

How to open the same project on one computer multiple times?

If I try it I get the message Looks like this project is already open.

I want to test a multiplayer without having to build on every change.

Some solutions I found are custom scripts that build and run multiple instances, or duplicating the whole project folder with a different name and symlink/hardlink/shortcut Assets folder but all of this feels redundant.

For Windows:

UnityProjectCloner makes a separate clone project that it shares files with over hard links. The solution below won’t work on Windows due to files being in use and resolving that with “closing handles” is unpractical.

For Linux:

Deleting <project>/Temp/UnityLockfile after opening an instance makes opening another instance possible.

When modifying the scene hit save to get the changes in other project instances.

Tested with 4 instances at the same time in play mode.

Note:

After closing an instance Unity deletes the Temp folder so various errors start to show in remaining instances like on saving, to resolve you should recreate the Temp folder.