Since allowing multiple author lines would require significant rewrites to the Library software, I would prefer any co-authors be listed in a history line. This ensures that the co-authorship is documented and that the co-author has a library account and has affirmed the CA.
Example:
0 !HISTORY XXXX-XX-XX [co-author] Co-authored part
Example:
0 !HISTORY XXXX-XX-XX [co-author] Co-authored part