PM (if required)
If you change your form, so the redirect points to a new (internal) page, lets say external.php.
set the redirect url as follows... /external.php?popup=true
create a new page called external.php, add the following code within the <head></head> tags
<?php
if (isset($_GET['popup'])) { ?>
<script>
var win = window.open("http://google.com", '_blank');
win.focus();
</script>
<?php } ?>
changing the url to the one you need
This should work as long as the browser doesn;t have popups blocked