Issue with multiple author templates

To confirm — both of you are using the template tags in your theme?

However this second version which is basically the same template is not showing the post on the co-authors page.

This sounds like your theme uses an alternate query to generate the list of co-author posts. Can you share what’s inside of your author.php template using pastebin?

It looks like you have something like the following in both templates:

$args = array(
          'view' => 'grid-small',
          'cat' => $cat,
          'link' => $link,
          'author' => $user_id,
          'post_type' => 'post',
          'ignore_sticky_posts' => true,
          'posts_per_page' => 3
);
dp_section_box($args);

This means the posts in those sections are limited to posts where post_author == $user_id. Because Co-Authors Plus stores byline information as taxonomy terms, I suspect that any post where the user is the second author is missing from your list.

I’m not sure how dp_section_box() works, but you might be able to pass a tax query to it. If that’s the case, it would look something like this:

$args = array(
          'view' => 'grid-small',
          'cat' => $cat,
          'link' => $link,
          'tax_query' => array(
                    array(
                              'taxonomy' => 'author',
                              'term' => get_queried_object()->user_login,
                              'field' => 'name',
                    )
          ),
          'post_type' => 'post',
          'ignore_sticky_posts' => true,
          'posts_per_page' => 3
);
dp_section_box($args);