Populate input from another form

I have a form

<form action="CRTP_Query.php" id="CRTP_Query" method="post" 
target="_CRTP"><input value="Query" form="CRTP_Query" type="submit"></form>

OnSubmit, I want to include data from another form (form="DPRform").

<input name="DPRsurname" type="text" form="DPRform" size="20" 
value="<?php echo stripslashes($_POST["DPRsurname"]);?>">

I should use a hidden identical field and use form="CRTP_Query":

<input name="DPRsurname" type="hidden" form="CRTP_Query" value="<?php 
echo stripslashes($_POST["DPRsurname"]);?>">

But I have no idea how to populate the hidden field with the data from 
the viewable field. PHP cannot do this onsubmit, can it?

Anyone have an example to show me please?

Do I need to use jquery?