Custom Orderby

functions.php

function theme_modify_the_main_query( $query ) {

	if ( isset( $_GET['orderby'] ) && $_GET['orderby'] == 'price' ) {
		// Modify the main query
		if( ! is_admin() && $query->is_main_query() ) {

            $args = array(
                    'relation' => 'AND',
                    'query_one' => array(
                        'key' => '_stock_status',

                    ),
                    'query_two' => array(
                        'key' => '_price',

                    ),
                'orderby' => array(
                    'query_one' => 'ASC',
                    'query_two' => 'ASC',

            ));

		    $query->set( 'meta_query', $args );
	      $query->set( 'order', 'ASC' );
        }
	}
}
add_action( 'pre_get_posts', 'theme_modify_the_main_query' );