How to get one form_input to show up in another?

jQuery is your best friend.
Load the jQuery library in your page's head section.
Then, at the bottom of your page (preferrably after the </body> tag):

Code:

<script>
$(document).ready(function() {
  $('#input1').keyup(function() {
      var text = $('#input1').val();
      $('#input2').val(text);
  });
});
</script>

In this example, "input1" is the id attribute of the input where the user is typing text, and "input2" is the id of the second field where you want to show the text that is being typed.
More info: https://api.jquery.com/keyup/