Calling a Controller function in another Controller in CodeIgniter

just load user controller as library from your controller

function __construct(){
     parent::__construct();
     $this->load->library('../controllers/user');
}

Now, call this function of user controller any where in your controller,

$this->user->logged_user_only();

Why not extend the controllers so the login method is within a MY controller (within the core folder of your application) and all your other controllers extend this. For example you could have:

class MY_Controller extends CI_Controller {
    public function is_logged()
    {
        //Your code here
    }
}

and your main controllers could then extend this as follows:

class Home_Controller extends MY_Controller {
    public function show_home()
    {
         if (!$this->is_logged()) {
           return false;
         }
    }
}

For further information visit: Creating Core System Classes

New link is here: https://www.codeigniter.com/user_guide/general/core_classes.html?highlight=core%20classes

Calling a controller from another is not possible with CI and not recommended.

Either move your logged_user_only into a helper or even better a core controller that you extend all of your controllers from (MY_Controller) see http://codeigniter.com/wiki/MY_Controller_-_how_to_extend_the_CI_Controller/