Wtile looking for the featcher running Joplin in two instances I did find a solution for me in the web.
The solution can be applied in Windows on different kinds of programms, also for Joplin.
But be careful there could be some severe syncing problems when not applied the right way. Maybe some of the Joplin Pros can give more Infos about that.
Nevertheless you have to create a 2nd User in Windows. Than you just logging to your Windows user profile you always use and hold shift while right kicking on the Joplin icon on the desktop. Here you will see "Run Instance with a different user".
Wola, now you can use it parallel. I am using one instance in darkmode, so I do not mix anything up. So again, be careful that you do not get any syncing problems. I for myself had no problems. I did change the settings for the 2nd profile so that I have to manually sync and try to work on the main profile, only.