Looking at the query being submitted for a co-author's archive page, we can see that we're looking up the taxonomy term ID (in this case, 57) as opposed to the co-author ID (in this case, 111). We might be able to insert a custom post counter if we knew how to relate these two numbers for the purpose of looping through a custom query.
SELECT SQL_CALC_FOUND_ROWS cw_posts.ID FROM cw_posts LEFT JOIN cw_term_relationships ON (cw_posts.ID = cw_term_relationships.object_id) LEFT JOIN cw_term_taxonomy ON ( cw_term_relationships.term_taxonomy_id = cw_term_taxonomy.term_taxonomy_id ) WHERE 1=1 AND (( (cw_term_taxonomy.taxonomy = 'author' AND cw_term_taxonomy.term_id = '57'))) AND cw_posts.post_type = 'listing' AND (cw_posts.post_status = 'publish' OR cw_posts.post_status = 'private') GROUP BY cw_posts.ID HAVING MAX( IF( cw_term_taxonomy.taxonomy = 'author', IF( cw_term_taxonomy.term_id = '57',2,1 ),0 ) ) <> 1 ORDER BY cw_posts.post_date DESC LIMIT 0, 12