Multiple composers

[simoncn]

FLAC files use Ogg/Vorbis comments for tagging. Multiple composers can be tagged using multiple COMPOSER= comments, for example:

COMPOSER=Adler, Richard
COMPOSER=Ross, Jerry

Some tagging programs support this by defining a separator such as ';' or '\\' that you can use in the Composer value to create the tags shown above. Other tagging programs provide an explicit selection in the UI (such as a "plus" button) to add multiple values for a selected tag.

I suggest you contact the author of the Meta tagging program to ask if it has this capability. If it doesn't, the Metadatics tagging program for macOS provides this capability using a "plus" button.