The goal is to first try to order by the custom meta field and if its not found then do order by the post date. Here is the query I did so far, but its not working.

$args = array( 
    'post_type' => 'xyz',
    'posts_per_page' => 5,
  'meta_query' => array(
    'relation' => 'AND',
    'query_one' => array(
      'key' => 'custom_meta',
      'compare' => 'EXISTS', 
    ), 
    'query_two' => array(
    'key' => 'post_date',
    ),
  ),
  'orderby' => array( 
    'query_one' => 'DESC',
    'query_two' => 'DESC',
  ),
  );
  $the_query = new WP_Query( $args );