Order query by meta_value with multiple custom fields

You can use meta_query to get only posts with display_submenuexpositions = true and order them by order_submenuexpositions.

$args = array(
    'orderby' => 'meta_value',
    'meta_key' => 'order_submenuexpositions',
    'meta_query' => array(
        array(
            'key' => 'display_submenuexpositions',
            'value' => 'true',
            'compare' => 'LIKE'
        )
    )
);
$query_posts( $args );