Replace line 30 of content.php which is
<span class="fn"><?php echo the_author_posts_link(); ?></span>
with
<span class="fn"><?php if ( function_exists( 'coauthors_posts_links' ) ) { coauthors_posts_links(); } else { the_author_posts_link(); } ?></span>
Another way if you want without hyperlinks, try pasting the below after line 156 of content.php
<?php if ( function_exists( 'coauthors' ) ) { coauthors(); } else { the_author(); } ?>