Currently I have this but it still puts the published date first, unless I misunderstood you and did something wrong of course…
case 'concertdatum2':
$wp_query_args['meta_query'] = array(
'datum_clause' => array('meta_key' => 'datum', 'compare' => 'EXISTS')
);
$wp_query_args['orderby'] = array('datum_clause' => 'DESC', 'date' => 'DESC');
break;