Some music I download has multiple artist tagged as ARTIST, and I was wondering if there is a way to automatically move them to MIXARTIST.
You can use the info from this thread to accomplish this.
Just update the destination field to what you prefer (%mixartist% perhaps?)
Note that it does appear from your examples that several have multiple artists split into separate fields, that is indicated by the double backslash character. You can combine these before you run the above action first, using the Action(Quick) function Merge Duplicate Fields.
I don't want to move the main (first) artist to MIXARTIST, I want to move the featuring (second, third, etc) artists to MIXARTIST when it is formatted like my screenshot.
So this:
ARTIST: Charli XCX\Kim Petras\Tommy Cash
Would become:
ARIST: Charli XCX
MIXARTIST: Kim Petras;Tommy Cash
Sorry I wasn't more clear the first time!
That is what that previous thread discusses. But you would have to make some adjustments, depending on where you want your data to go.
Try this for your example;
Source: %ARTIST%
Guessing pattern: %ARTIST%%MIXARTIST%
In this new example, you show that you want the names in Mixartist to be separated by a colon rather than the backslash.You could run a separate action here to replace that character if the is what you want.
Oh sorry I misunderstood! Let me try that!
Update: Nothing happened. To clarify, I am making a "Guess values" action?
You can do that once you nail down what you want to do. I prefer to use the Actions (Quick) menu to get right at it. Can you copy/paste the entire contents of that Title field here? I can try this with exactly what you have. Sometimes there are hidden or alternate characters that can influence this.
Ah thank you!
Charli XCX\\Kim Petras\\Tommy Cash
Also if this helps!
To get the ARTIST separated, try an action of the type Guess value
Source: $regexp($meta_sep(artist,\\),(.*?)\\\\(.*),$1==$2)
Target string: %artist%==%mixartist%
It works!! Thank you so much for saving hours of my life!!! 
I ran into a new issue and I was wondering if you can help. Is there anyway to make it output to mixartist as artist;artist;artist instead of artist\\artist\\artist? I thought they were basically the same so I didn't ask, but I noticed MusicBee isn't reading the \\ and lumping it all together as one artist, where ; will split things apart. I tried doing a replace, but it doesn't work, I think because \\ means its in different tags technically?
The backslash is the way that MP3tag represents multi-value fields in a single UI element.
Check the extended tags dialogue if you find several fields of the same type.
It is astonishing that MusicBee also reads the multi-value fields and puts all the fields together.
If you want to see als multivalue fields in 1 field, try
and use the semicolon as separator.
Thanks! What an easy fix!
Both mp3tag and MusicBee support multi-value fields. They just use different characters to show them. Mp3tag uses the double backslash, MusicBee uses the semicolon. But the tags are already separated in both programs.