class ParentController extends CI_Controller {

    function __construct()
    {
        parent::__construct();
        //your constructor code here..
    }
}

class ClassA extends ParentController {
    function __construct()
    {
        parent::__construct();
    }
    //your first controller
}

class ClassB extends ParentController{
    function __construct()
    {
        parent::__construct();
    }
    //Your second controller
}