According to https://codex.wordpress.org/Class_Reference/WP_Query#Order_.26_Orderby_Parameters you want to specify it similar to the following code example:

    $args = array( 	'orderby' => array( 'title' => 'DESC', 'menu_order' => 'ASC' ) ); $query = new WP_Query( $args );

This is supported from WordPress 4.0. 

For custom fields (post meta) you will need an added meta query, as detailed on https://make.wordpress.org/core/2015/03/30/query-improvements-in-wp-4-2-orderby-and-meta_query

E.g:

    $q = new WP_Query( array ('meta_query' => array(
        'relation' => 'AND',
        'state_clause' => array(
            'key' => 'state',
            'value' => 'Wisconsin',
        ),
        'city_clause' => array(
            'key' => 'city',
            'compare' => 'EXISTS',
        ), 
    ),
    'orderby' => array(
        'city_clause' => 'ASC',
        'state_clause' => 'DESC',
    ),
    ) );

This is supported since 4.2.