I wrote a custom validation...

function shopping_form_alter($form_id, &$form) {
if (strcmp('uc_cart_checkout_form', $form_id) != 0) return;
$form['#validate'] = array('credit_check_expdate' => array());
}

But at the end of my validation, I called
uc_cart_checkout_form_validate($form_id, $form_values)

Is this OK way to do this, could I have changed my form alter so BOTH validations would have been called?
JIm