This is a WordPress plugin to add as much authors to a post as you wish.
Like always: copy to wp-content/plugins/additional-authors and activate in backend.
Best practice: Use do_action('additional_authors_the_authors') to render. Optional second parameter can be the post_id if not in context of loop.
You can overwrite templates by copying from plugins folder templates to your theme/plugin-parts/*.
If you don't want additional authors to show up when using has_published_posts argument than you can set ignore_published_as_additional_author to TRUE to ignore them.