<script type="text/javascript">

     function textChange(id, fn) {

         var textarea = document.getElementById(id);

         if ("\v" == "v") {

             textarea.onpropertychange = fn;

         }

         else {

             textarea.addEventListener("input", fn, false);

         }

     }

     function SetTxt2() {

         document.getElementById("txt2").value = document.getElementById("txt1").value;

     }

     function SetTxt1() {

         document.getElementById("txt1").value = document.getElementById("txt2").value;

     }

     onload = function () {

         textChange("txt1", SetTxt2);

         textChange("txt2", SetTxt1);

     }

 </script>

 <input type="text" name="" value="" id="txt1" /><br />

 <input type="text" name="" value="" id="txt2" />