Cacher is the code snippet organizer for pro developers

<?
//How to orderby using multiple meta values
$query = new WP_Query( 
  array(
    'post_type' => 'sessions', 
    'posts_per_page' => -1,
    'meta_query' => array(
      'relation' => 'AND',
        'hour_clause' => array(
          'key' => 'start_hour',
          'compare' => 'EXISTS',
        ),
        'min_clause' => array(
          'key' => 'start_min',
          'compare' => 'EXISTS',
        ), 
    ),
    'orderby' => array(
      'hour_clause' => 'ASC',
      'min_clause' => 'ASC',
    ),
  ) 
);
?>