WP_Query - multiple custom post types and sort by custom meta
use orderby
instead of order_by
$warsztaty_q = new WP_Query(array( 'post_type' => array('kalendarium', 'warsztaty'), 'orderby' => 'meta_value', 'meta_key' => 'data_start', 'order' => 'ASC'));
refer codex for more details