There is no way I know of where you can send one form to two pages.
But we can use two buttons
Code:
<form name="f1" method="POST" id="ss-form" action="https://docs.google.com/spreadsheet/formResponse?formkey=dE02TlBHQ2hnVVdkTE5ya0lBYXhaMlE6MQ&ifq">
<input type='submit' value='Send to thank you page' onclick="f1.action='ThankYou.php'; return true;">
<input type="submit" name="submit" value="Submit">
</form>
Notice the addition in red name="f1"