UPDATED: This Tag That Tag v5.4
For macOS 10.15 Catalina and later only. This applet will assist with swapping, copying, and appending data between two user-chosen tags in selected tracks or tracks in the selected playlist:
Swap - swap data between tags, ex: ARTISTCOMPOSER
Copy - copy data from one to another tag, ex: ARTIST->COMPOSER ARTIST
Append - append data from one tag to the end of another, ex: ARTIST->COMPOSER - ARTIST
Prepend - append data from one tag to the beginning of another, ex: ARTIST->ARTIST - COMPOSER
The latter three actions also provide an option to delete the info from the first tag after the copy. Read Me explains all.
More information for This Tag That Tag v 5.4 and download link is here.