When I load my personnal loader :

 
require_once "Zend/Loader.php";
require_once "Olfp/Loader.php";

with special dir all is ok :

 
class Olfp_Loader extends Zend_Loader
{
    public static function loadClass($class, $dirs = null)
    {
        $dirs = array(
            './library/Client/Chu',
            './library'
        );

        parent::loadClass($class, $dirs);
    }

    public static function autoload($class)
    {
       try {
            self::loadClass($class);
            return $class;
        } catch (Exception $e) {
            return false;
        }
    }
}

when a write :

 
Olfp_Debug::name();

ouput is : "Olfp_Debug chu", so my special class is really loaded, it's ok.

but when a write just after this :

 
Zend_Session::start();

I've got an error :

Fatal error: Cannot redeclare class Zend_Session in C:\wamp\www\testPf\library\Zend\Session.php on line 49