turns out it needed to be post_date instead of simply date, this is the working code.
case 'concertdatum':
$wp_query_args['meta_query'] = array(
'datum_clause' => array('key' => 'datum','compare' => 'EXISTS')
);
$wp_query_args['orderby'] = array('datum_clause' => 'DESC', 'post_date' => 'DESC');
break;
For some odd reason however, the mega menu that my theme uses refuses to sort the posts like I request it to, but that’s a different question not for this site 😉
