Zend_Form: how to check 2 fields are identical

$password = new Zend_Form_Element_Password('password', array(
		'validators'=> array(
			'Alnum',
			array('StringLength', array(6,20))
			),
		'filters'	=> array('StringTrim'),
		'label'		=> 'Wachtwoord:'
		));

	$password->addFilter(new Ivo_Filters_Sha1Filter());

	$password2 = new Zend_Form_Element_Password('password', array(
		'validators'=> array(
			'Alnum',
			array('StringLength', array(6,20))
			),
		'filters'	=> array('StringTrim'),
		'required'	=> true,
		'label'		=> 'Wachtwoord:'
		));
	$password2->addFilter(new Ivo_Filters_Sha1Filter());

<?php

class Zend_Validate_IdenticalField extends Zend_Validate_Abstract {
  const NOT_MATCH = 'notMatch';
  const MISSING_FIELD_NAME = 'missingFieldName';
  const INVALID_FIELD_NAME = 'invalidFieldName';

  /**
   * @var array
  */
  protected $_messageTemplates = array(
    self::MISSING_FIELD_NAME  =>
      'DEVELOPMENT ERROR: Field name to match against was not provided.',
    self::INVALID_FIELD_NAME  =>
      'DEVELOPMENT ERROR: The field "%fieldName%" was not provided to match against.',
    self::NOT_MATCH =>
      'Does not match %fieldTitle%.'
  );

  /**
   * @var array
  */
  protected $_messageVariables = array(
    'fieldName' => '_fieldName',
    'fieldTitle' => '_fieldTitle'
  );

  /**
   * Name of the field as it appear in the $context array.
   *
   * @var string
   */
  protected $_fieldName;

  /**
   * Title of the field to display in an error message.
   *
   * If evaluates to false then will be set to $this->_fieldName.
   *
   * @var string
  */
  protected $_fieldTitle;

  /**
   * Sets validator options
   *
   * @param  string $fieldName
   * @param  string $fieldTitle
   * @return void
  */
  public function __construct($fieldName, $fieldTitle = null) {
    $this->setFieldName($fieldName);
    $this->setFieldTitle($fieldTitle);
  }

  /**
   * Returns the field name.
   *
   * @return string
  */
  public function getFieldName() {
    return $this->_fieldName;
  }

  /**
   * Sets the field name.
   *
   * @param  string $fieldName
   * @return Zend_Validate_IdenticalField Provides a fluent interface
  */
  public function setFieldName($fieldName) {
    $this->_fieldName = $fieldName;
    return $this;
  }

  /**
   * Returns the field title.
   *
   * @return integer
  */
  public function getFieldTitle() {
    return $this->_fieldTitle;
  }

  /**
   * Sets the field title.
   *
   * @param  string:null $fieldTitle
   * @return Zend_Validate_IdenticalField Provides a fluent interface
  */
  public function setFieldTitle($fieldTitle = null) {
    $this->_fieldTitle = $fieldTitle ? $fieldTitle : $this->_fieldName;
    return $this;
  }

  /**
   * Defined by Zend_Validate_Interface
   *
   * Returns true if and only if a field name has been set, the field name is available in the
   * context, and the value of that field name matches the provided value.
   *
   * @param  string $value
   *
   * @return boolean
  */
  public function isValid($value, $context = null) {
    $this->_setValue($value);
    $field = $this->getFieldName();

    if (empty($field)) {
      $this->_error(self::MISSING_FIELD_NAME);
      return false;
    } elseif (!isset($context[$field])) {
      $this->_error(self::INVALID_FIELD_NAME);
      return false;
    } elseif (is_array($context)) {
      if ($value == $context[$field]) {
        return true;
      }
    } elseif (is_string($context) && ($value == $context)) {
      return true;
    }
    $this->_error(self::NOT_MATCH);
    return false;
  }
}
?>

$form->addElement('password', 'elementOne');
$form->addElement('password', 'elementTwo', array(
    'validators' => array(
        array('identical', false, array('token' => 'elementOne'))
    )
));

$password_2->addValidator('identical', false, $this->_request->getPost('password'));