Exactly that is the problem. 1 instance system-wide is enforced. I can't fathom why for the life of me.
I would also be happy to have more copies of Mp3tag running, for several reasons.
I did not find an easy/quick solution for the first reason (i use a different setup in Tag Panel and in Fields Table for easy music and classical music).
But i use a workaround for the other reason, to change a predetermined sets of files quickly: manually created playlistes in foobar. You can quickly drag the contents of a playlist from the foobar to the Mp3tag with the mouse. If you hold down the control key, the playlist in the foobar will not be emptied either. (Although, this is, how files are added in Mp3tag, so you have to empty Mp3tag table first.) And when you drag a list, the content in Mp3tag is always updated as well.
But yes, for this second purpose a "tabbed" Mp3tag would be an easier variant. When we switch tabs, Mp3tag could update the files the same way, as when we drag them with the mouse. It takes the same time, but we could save the time of dragging with mouse.
Hmm... Besides the tabs, maybe even 2 windows, like in Total Commander...
Music Commander 
you sound like steve jobs telling everyone jow to hold their iphone right. “it’s not a design flaw, you’re just doing it wrong!” 
anyhow, 6 SSD, NAS, cloud. everything holds music for different purposes.
I’ve been working on them with multiple instances for days now and it works just fine. and faster.
despite the SATA interfaces.
while mp3tag re-loads 100k files to display tags that have been modified outside of mp3tag, I’m happily processing files in another instance of mp3tag from SSD2 and SSD3.
I understand some of your points though.
I'm sorry you're irritated. The concept of a WISHLIST is exactly that. A Wish List. It's not our job to overthink the dangers, nor should dreamers be concerned with irritating others.
Sometimes one just throws out an idea. Maybe it's wild & crazy, maybe it has potential. Maybe someone says "I found a different way to do that". Maybe the creator says "I'll give it some thought". Maybe the creator says "That won't work because...". But nothing happens without discussion, and isn't that what a WISH list is, just a discussion of desires, of dreams, of ideas?
Your contributions are greatly appreciated here, more than you could know, but please don't tell us what to wish for.
Here's a PowerShell script which closes the "SingleInstance" handle for all open MP3Tag instances via SysInternals handle.exe. PS syntax drove me crazy but then again it's my first time writing anything in PS. So it might not look elegant, but it works. Feel free to improve and repost. Later I might turn it to a proper launcher with AHK or NSIS.
# **Use at your own risk!**
#
# Has to be run as administrator, otherwise handle.exe cannot close the SingleInstance handle
#
# ADJUST PATH
# Change the path to yours or put handle.exe somewhere in your %PATH% and remove Y:\
$handleoutput = (Y:\handle.exe -nobanner -p mp3tag.exe)
$blocks = ($handleoutput -join " " -split '-{20,}')
for($i = 1; $i -lt $blocks.length; $i++) {
$tmp1 = ($blocks[$i] | Select-String -Pattern 'pid:\s+(\d+)')
if ($tmp1 -ne '') {
$hpid = $tmp1.Matches.GetValue(0) -replace 'pid:\s+', ''
$tmp2 = ($blocks[$i] | Select-String '(\S+?): Section\s+\S+?SSingleInstance_MMF_Mp3tag')
if ($tmp2 -ne '') {
$hhnd = $tmp2.Matches.GetValue(0) -replace ':.+', ''
}
}
if ($hpid -ne '' -and $hhnd -ne '') {
write-output "Process ID: $hpid, Handle ID: $hhnd"
# ADJUST PATH
Y:\handle.exe -nobanner -p $hpid -c $hhnd -y
}
}
I manually close the handle but I'll give this a go later. thanks mate
This can be solved a little more simply:
Put this 3 lines in a new batch file and call it CloseMp3tagSingleInstanceHandle.cmd
(or whatever you like):
SET MyHandle=C:\Temp\MyHandle\handle.exe
FOR /F "tokens=3,6 delims=: " %%I IN ('%MyHandle% SingleInstance_MMF_Mp3tag -nobanner') DO %MyHandle% -c %%J -y -p %%I -nobanner
PAUSE
Adjust the first line with the existing path where you save the following handle.exe tool.
You need to download the mentioned handle.exe command line tool once from here:
Unzip handle.exe from the handle.zip archive and put it in any directory you want.
Just edit the first line in the above batch file with this path to your handle.exe. In my example, I have placed handle.exe in the newly created directory C:\Temp\MyHandle
If Mp3tag is already running, executing the batch with Administrator rights will show you something like this:
If the SingleInstance-Handle is already closed or Mp3tag is not running at all, you get something like this:
You have to start your CloseMp3tagSingleInstanceHandle.cmd with Administrator rights.
If you don't want to see any output, just remove the 3rd line PAUSE in your batch file.
IMPORTANT:
If you run this batch to use multiple Mp3tag instances at the same time, you risk to get unexpected results. I do NOT recommend it as @florian has not built Mp3tag for this use case!
Be aware and careful, always backup your data!
I knew it, having to parse the whole thing was overkill 
Your command is much more concise, and gives it directly. Thanks!
Maybe a little improvement from me, too:
%MyHandle% -p mp3tag.exe SingleInstance_MMF_Mp3tag -nobanner
makes handle.exe to go over only mp3tag.exe's handles, instead of all processes.