I have a much different reason for wanting to have multiple instances, not tabs but separate instances with their own .ini and config files: Keeping music configuration separate from movie configuration.
"Movie configuration" you ask? MP3Tag is unsurprisingly also one of the best tools to edit MP4 and MKV files. I've set up a genres list specific to movies, which has none in common with music genres, and additional fields in tag panel which make sense only for movies, such as actors list, IMDB rating, TMDB id, etc.
Keeping 2 configurations in 1 instance is not practical though. Like others said, VM, Sandboxie, etc. do work, but why MP3Tag enforces single instance is beyond me. I remember people asking in older threads "but how would the instances synchronize between each other?" They shouldn't even try. And no "lock files" please, this is not a RDBMS on an SAP system. Even after updating a file MP3Tag doesn't keep files in exclusive lock mode, and it never should. It's not MP3Tag's duty to fix the user's lack of attention to which window he changes data in. After loading a folder an MP3tag, one could still edit the tags in another program and make the data loaded in MP3Tag out-of-date anyway. In fact I do it all the time, using Foobar2000 & MP3tag back and forth. There must be another reason why Florian doesn't want this because if up-to-dateness between multiple instances should be the reason, that's like having a text editor which wants to allow only 1 instance of itself whereas you could easily open another editor and edit the same file.