Write the data of an inputText into another inputText

If it is at runtime (without reloading the page) it is best to use JQuery:

 <script> 
                jQuery(document).ready(function () {
                    jQuery('#idCampoOrigem').blur(function () { 
                        jQuery('#idCampoDestino').val(jQuery('#idCampoOrigem').val()); 
                    });
                });
            </script>