Can’t place two custom post types into the WordPress search query?

Try to drop that in your functions.php file:

function filter_query_for_search( $query ) {
    if ( isset( $query['s'] ) ) {
        $query['post_type'] = array('forum', 'topic', 'reply', 'any');
    }
    return $query;
}

add_filter('request', 'filter_query_for_search', 1);

that would hook into the query just before its being executed.