gistfile1.html

<form onsubmit="return makeFieldsTheSameValue();">
  
  <!-- 
    It's important that your input fields have unique ID attributes,
    otherwise we won't be able to reference them easily via JavaScript. 
  -->
  <label for="field_one">Field One</label>
  <input type="text" name="field_one" value="" id="field_one">

  <label for="field_two">Field Two</label>
  <input type="text" name="field_two" value="" id="field_two">
  
  <input type="submit" name="some_name" value="Submit" id="some_name">
</form>

<script type="text/javascript" charset="utf-8">

  // The key function is document.getElementById — using that, we
  // can manipulate almost anything we need to.
  function makeFieldsTheSameValue() {
    
    
    // Assuming we nee need the other field to be set the same as
    // an existing field, we can get the existing value...
    fieldOneValue = document.getElementById('field_one').value
    
    // And then ensure that our other field has the same
    document.getElementById('field_two').value = fieldOneValue
    
    // Finally, return true to ensure that the form submits successfully
    return true;    
  }
</script>