Symptoms:
- When more than one instance of Unity references the same project folder, all the other instances of Unity (apart from the initial one) show an error message saying, "Multiple Unity instances cannot open the same project."
Cause:
This error appears because Unity is trying to protect the project from going out of sync. For example, multiple changes to the same could object cause conflicts within the project's references.
There are situations where having multiple instances of Unity running the same project is helpful. One of these is when testing multiplayer games, where running a version of the project as the host and one as the client allows in-Editor debugging for both host and client.
Resolution:
THIS IS NOT SUPPORTED. USE AT YOUR OWN RISK
If you are happy to prevent this from happening, you can create a workaround by creating a symbolic link between the project's Assets and ProjectSettings folders.
This allows you to keep all Editor instances in sync when making edits in the initial instance of Unity. But please keep in mind that we don't recommend making edits in the other Unity instances.
- cd <duplicated_project_folder>
- mklink /D Assets <original_project_folder>\Assets
In the example project I had, I created a symbolic link to the Assets folder in my new project using this link in Command Prompt:
mklink /D "x:\Personal Projects\Tanking_Dub02\Assets" "x:\Personal Projects\Tanking\Assets"
A translation of this would be:
mklink /D <location of the symbolic link in your duplicate project> <Original file>
More Information:
https://www.howtogeek.com/howto/16226/complete-guide-to-symbolic-links-symlinks-on-windows-or-linux/