protected function _prepareCollection()
{
    $collection = Mage::getResourceModel('test_testing/order_collection');
    $collection->setProductFilter($this->_product->getId())->setParentItemIdFilter();
    $coreResource = Mage::getSingleton('core/resource');
    $collection->getSelect()
               ->joinLeft(array('billing_address' => $coreResource->getTableName('sales_flat_order_address')),
                    '`sales_order`.`billing_address_id` = `billing_address`.`entity_id`',
                    array('customer_firstname'  =>  'billing_address.firstname',
                        'customer_lastname'     =>  'billing_address.lastname',
                        'customer_email'        =>  'billing_address.email',
                    ));

    $collection->getSelect()->limit(2);
    $this->setCollection($collection);
    return parent::_prepareCollection();
}