Order by two meta values using meta_query

gistfile1.txt

$meta_query = array(
    'relation'  => 'OR',
    'featured' => array( // give the first meta key array an array key
        'key'       => 'is_featured',
        'compare'   => 'EXISTS',
        'type'      => 'NUMERIC'
    ),
    'delivery_date' => array( // give the second meta key array an array key
        'key'       => 'to_be_delivered',
        'compare'   => 'EXISTS',
        'type'     => 'NUMERIC',
    ),
);

// order by using the meta array keys to reference them
// first order by 'delivery_date' value, then order by 'featured' value
$orderby = array(
    'delivery_date'    => 'DESC',
    'featured'         => 'DESC',
);

$query->set( 'meta_query', $meta_query );
$query->set( 'orderby', $orderby );