wp_query_orderby_custom_fields.php

function advwp_edit_event_order( $orderby ) {
  // First sort by start date, but then internally sort
  // by start time - starting with the earliest.
  return 'mt1.meta_value, mt2.meta_value ASC';
}

$args = array(
  
  // fetch only "events""
  'post_type' => 'event',
  
  // we can't orderby a meta_value if the
  // key isn't present ... - even if we 
  // aren't actually using 'orderby' directly
  // in the query.
  'meta_key' => 'advwp_start_date',
  
  // We define multiple keys using 'meta_query'
  'meta_query' => array(
    
    // the first key, or mt1
    array(
      'key' => 'advwp_start_date'
    ),
    
    // the second key, or mt2
    array(
      'key' => 'advwp_start_time'
    )
  )
);

add_filter( 'posts_orderby', 'advwp_edit_event_order' );
$the_query = new WP_Query( $args );
remove_filter( 'posts_orderby', 'advwp_edit_event_order' );