Literally seconds after updating this post I found the solution….again, Must have looked at this previously and not entered the details correctly as I tried this one before. 🙂
add_action('pre_get_posts', 'add_special_sort', 11, 1);
function add_special_sort($query){
// Bail if not the main "hidden" query, as opposed to a 'new WP_Query()' call
if(!$query->is_main_query())
return;
$query->set('meta_query', array(
'relation' => 'OR',
array(
'key' => 'featured_listing',
'compare' => 'NOT EXISTS',
),
array(
'key' => 'featured_listing',
'compare' => 'EXISTS',
)
));
//$query->set('meta_key', NULL);
$query->set('orderby', array('meta_value_num' => 'DESC', 'date' => 'DESC'));
}