[monolithed]

lolka84 где ваш код?

На вскидку:

<form name="forma">
    <input type="text" />
    <input type="text" />
</form>

<script type="text/javascript">
window.onload = function(){
    var event = ['keyup', 'change'];
    for(i=0; i<event.length; i++){
       document.forms['forma']['on'+event[i]] = function(i){
           return function(){
               this.elements[1].value = (this.elements[0].value == '') ? '' : eval(this.elements[0].value);
           };
      }(i);
    }
};
</script>