Form field concatenation

OVERVIEW:
Need to concatenate two form fields and pass the result with the form submission.

PSEUDOCODE EXAMPLE:
<FORM name="frm_Join" METHOD="POST" ACTION="http://www.aweber.com/scripts/newlead.pl">

      <input type="text" name="custom txt_FirstName" id="firstname">
      <input type="text" name="custom txt_LastName" id="lastname">

      <SCRIPT LANGUAGE="JavaScript">
            <!-- Start
            function showFullName()
            {
            var jFirstName = document.getElementById("firstname").value
            var jLastName  = document.getElementById("lastname").value
            var jFullName  = jFirstName + " " + jLastName
            document.frm_AffJoin.name.value = jFullName
            }
      // End -->
      </SCRIPT>

      <INPUT TYPE="HIDDEN" NAME="FullName" value="FirstName LastName">

</form>

SOLUTION REQUIRED:
For the hidden FullName field to pass a concatenation of FirstName+" "+LastName when the form is submitted.

CONDITIONS:
The name of the FirstName and LastName input fields has to be two words, starting with "custom". That is a requirement of the Perl target script. That's why I'm trying to use ID.

EXPLANATION:
My JavaScript is almost non-existent, but I have tried to reach a solution by concatenating the FirstName and LastName fields in javascript, and pass the jFullName variable to the hidden field, but I don't know how to do it and everything I have tried fails. I've cut down the code just to show clearly what I am trying to do.

Thanks in advance for any help.
 - Bill H.