flash & flashSession message

In my application, flash messages can be output by {{ content() }}, but CANNNOT output by {{flash.output()}}

and flashSession messages can be output by {{flashSession.output()}}, but CANNOT output by {{content}}!

How to output them both by {{content()}}?

$this->flash->error("Problem does not exist " . $id);
            return $this->dispatcher->forward(
                array(
                    "action" => "list"
                )
            );
...

$this->flashSession->success("created！");
$this->response->redirect('problems/list');