OderBy two Meta

Hello everyone,

This is my first post here, I hoe this topic will be useful for many other people.

Here is my problem : I have posts with custom fields information, and I would like to sort them with 2 meta_value.
Here is my not working code :

<?php
$idf_dept = array ('75', '77', '78', '91', '92', '93', '94', ' 95'); // the list "departements" I want OR don't want depending on the user choice
 $mapposts = new WP_Query( array('post_status' => 'publish',
								'post_type' => 'post',
								'category_name' => 'en_cours',
								'meta_query' => array(
														array(
															'key'     => 'dept_nb_for_the_order',
															'value'   => $idf_dept,
															'compare' => $compare,
															'orderby' => 'meta_value',
															'order' => ASC
														),
														array(
															'key'     => 'city_name_for_the_order',
															'orderby' => 'meta_value',
															'order' => ASC
														)),
								) );
?>

I’ve seen a lot of solutions, but not working for me… some are too old maybe.

I hope one of you have an answer, I’m getting mad on this one !!
Thanks a lot ??