in the server validation behavior, click the lightning bolt icon next to server variable and select the first form field
the code entered for that selection will look like this:
<?php echo((isset($_POST["field1"]))?$_POST["field1"]:"") ?>
modify that to concatenate the second form element:
<?php echo(((isset($_POST["field1"]))?$_POST["field1"]:"")."".((isset($_POST["field2"]))?$_POST["field2"]:"")) ?>
this will mean the value being validated is the two elements combined.