Multiple sorting criteries (order by)

The solution is old as time is.

And it was already described earlier.
Place in functions.php

class iWC_Orderby_Stock_Status
{

 public function __construct()
 {
  // Check if WooCommerce is active
  if (in_array('woocommerce/woocommerce.php', apply_filters('active_plugins', get_option('active_plugins')))) {
   add_filter('posts_clauses', array($this, 'order_by_stock_status'), 2000);
  }
 }

 public function order_by_stock_status($posts_clauses)
 {
  global $wpdb;
  // only change query on WooCommerce loops
  if (is_woocommerce() && (is_shop() || is_product_category() || is_product_tag())) {
   $posts_clauses['join'] .= " INNER JOIN $wpdb->postmeta istockstatus ON ($wpdb->posts.ID = istockstatus.post_id) ";
   $posts_clauses['orderby'] = " istockstatus.meta_value ASC, " . $posts_clauses['orderby'];
   $posts_clauses['where'] = " AND istockstatus.meta_key = '_stock_status' AND istockstatus.meta_value <> '' " . $posts_clauses['where'];
  }
  return $posts_clauses;
 }
}

new iWC_Orderby_Stock_Status;

And all is in the bag. This solution fits my needs.