Florian, I would like a -solo commandline option that causes the .exe to run a separate instance regardless of any instance already running. If this is allowed only on a portable install, OK.
I pledge 25e for this. Users, please add your pledges here.
The ability to run multiple instances of Mp3tag has been requested in the past. Personally, I'd have zero use for this functionality.
Then I take it you do not have anything to contribute to this discussion. 
Anyone who does - please do!
Ok, let's pretend that this hasn't been discussed in the past, or that you don't know how to use the forum search function. 
Why do you think you need this?
I guess you don't know how to use the forum search function. Try here: /t/15071/1
So apparently you had nothing to add to that discussion, which is all of five weeks old. You thought it better to start a new one.
You got it.