geocraft | Generate minecraft maps of real locations using UK LIDAR

Assuming that $listing is in the format of a WP $post object, right after

$to = get_the_author_meta('user_email', $post_author);

add

$to .= ',' get_post_meta($listing->ID, 'geocraft_meta_email', true);

That way you are sending one email to two people. It would be cleaner to send a total of 3 separate emails, but it's not clear whether or not you have access to the directory listing person's name. You really need database access in order to do this sort of work.

To make sure you don't accidentally email real users, before you make any changes, copy your whole website to a staging/dev site. You can then use phpMyAdmin to change every instance of geocraft_meta_email to your own test email address. I like to use Gmail because if you have an address like myname@gmail.com you can actually email myname+anystringyoulike@gmail.com - that way you still receive the actual emails, but it lets you identify exactly which email came through.