access parent class on class overwrites in magento, from a sibling

It is required to overwrite customer_resource class. I added required config.xml options for resource rewriting, but when overwritten class method calls parent class method from itself, it actually called the parent method from which I extend to, nor the parent of Mage_Customer_Model_Resource_Customer:

  class My_Plugin_Model_Resource_Customer extends Mage_Customer_Model_Resource_Customer 
    {

    protected function _beforeSave(Varien_Object $customer){
        parent::_beforeSave($customer);
    ...
        }
    }

So, how to call parent method correctly in this case ? some kind of double downcasting is possible in php ?