In this cases you can try some old school php.
// insert at the beggining of home.php controller
require_once(dirname(__FILE__)."/product.php"); // the controller route.
Then, you'll have something like:
Class Home extends CI_Controller
{
public function __construct()
{
parent::__construct();
$this->product = new Product();
...
}
...
// usage example
public function addProduct($data)
{
$this->product->add($data);
}
}
And then just use the controller's methods as you like.