It's possible to tag multiple artists formatted for MusicBee using the MusicBrainz Picard tagger. Currently Display Artist, Guest Artist, Remixer, and Performers are supported.
Many thanks to Steven and to Picard developers, especially Sophist, for the updates that make this work.
To use scripts in Picard, open the Options screen and go to Advanced > Scripting, then check the box at the top. Copy and paste the code. You can leave the formatting as shown here or put it all on one line, it doesn't matter.
Display Artist and Guest Artist Edit
This script will find all tracks with multiple artists (not including Performer credits) in the MusicBrainz database and set Artist and Display Artist tags appropriately, plus tag feat. artists as Guest Artists. (If you're using ID3v2.3 tags, be sure that the "Join multiple ID3v2.3 tags with:" option is set to ";" rather than "/".)
$if($in(%artists%,;), $set(DISPLAY ARTIST,%artist%) $if($in(%artist%,feat.), $set(artist,$rreplace(%artist%,\\sfeat.*,)) $setmulti(GUEST ARTIST,$replace(%artists%,%artist%; ,)) , $copy(artist,artists) ) ,)
If you prefer to keep the original artist value and append the split artists, you can use $copymerge instead of $copy.
Remixer often needs no special treatment. If a Remixer value is in the MusicBrainz database, Picard will tag it and MusicBee will read it into the Multiple Artist Splitter.
For cases where there is no Remixer set in the MusicBrainz database, this script will catch any of them in the form "mix by" or "remix by" and set Remixer for you:
$if($in(%title%,mix by ), $if($not(%remixer%), $set(remixer,$rreplace(%title%,.*mix by ,)) $set(remixer,$replace(%remixer%,\),)) ,) ,)
It's more difficult to identify cases where the format is "Artist mix", since it could also be a remix done by the original artist with a name like "Club mix." For now, you'll have to tag those manually.
MusicBee now handles performer values in the Multiple Artist Splitter out of the box.
Picard Plugin Edit
Note: May or may not work with current version of Picard.
Forum member vzell and MusicBrainz developer Sophist created a plugin that will tag multiple artists in Picard in some useful ways.
You can find it here (right click and save as musicbee_compatibility.py) : https://raw.githubusercontent.com/Sophist-UK/sophist-picard-plugins/master/musicbee_compatibility.py