auto filling one field with the same value as a previous field?

I looked in the issue area for something that looked like it related to my question but I either don't know what terms to use or no one has really asked about it.

I have one select field called country where users need to pick their country. Then I have another field down farther was has to do with some engineering type stuff that has to have the same input as the country field. Is there a way that when they select a country, it pre-populates the other field below it?

I tried using this code here to at least ensure the same answer was typed in but it isn't working:

<?php
  if ($form_values['submitted_tree']['country'] != $form_values['submitted_tree']['preferred_wri_code']) {
    form_set_error('submitted][country', t('These country codes do not match.  They must match before you can submit your form.'));
  }
?>

Not sure why....

Becky