It might be useful to have multiple versions of MiddleVR installed: for example one stable version and one beta version.
MiddleVR’s installer will not allow you to have multiple installations at the same time.
What you can do though is copy (not rename, because this will mess with the installer) the "C:\Program Files (x86)\MiddleVR" folder of one version to "C:\Program Files (x86)\MiddleVR_beta" for example.
Then uninstall the current version: this should remove the "C:\Program Files (x86)\MiddleVR" folder but you should still have the MiddleVR_beta folder.
You should now be able to install the other MiddleVR version.
Then if you want to change between one version or the other, you should just change the PATH environment variable between the MiddleVR\bin and MiddleVR_beta\bin folders.
Make sure to restart Unity, MiddleVR config and any Windows file explorer that you are using to execute your application.