Make sure to check the Required Attribute. If you do not select the required attribute for the form field, you will be able to enter a space.
Are you saying you want to get the total of the two quantity fields? If so you can do simple math on the values entered.
for this example, the quantity form elements are named qty1 and qty2
<?php
if((isset($_POST['qty1']) && is_numeric($_POST['qty1'])) && (isset($_POST['qty2']) && is_numeric($_POST['qty2']))) {
$totalQTY = $_POST['qty1'] + $_POST['qty2']
echo(totalQTY);
}
?>