Yes you need to do that in the php file responsible for processing the form. Something like
var1 -dropdown
var2 - text field
P.S. same happened to me - forum submitted before I finished
PHP Code:
if (!empty($var2)) {
# use text field value
if (!is_numeric($var2))
$error = true;
# if field value is not a number throw an error, process it and redirect back to the form for re-submision
else
# if field value is a number
$var2 = intval($var2);
# do more php stuff
} else {
# if text field is empty use dropdown value
$var1 = intval($var1);
# do more php stuff
}