add the Universal Email server behavior to the page twice, once for each email.
then add an if statement around each one so that it will only send based on the recordset column value:
<?php if($row_recordsetName['stockColumn'] == "in stock") { ?>
<UE behavior for the Instock message>
<?php } ?>
<?php if($row_recordsetName['stockColumn'] == "out of stock") { ?>
<UE behavior for the out of stock message>
<?php } ?>