why not use .onkeyup?

<input id="input1"></input>
<input id="input2"></input>

var input1,input2;
input1= document.getElementById("input1");
input2 = document.getElementById("input2");
input1.onkeyup = function () { input2.value = input1.value; };
//now if you want to copy edits from input2 to input1
// input2.onkeyup = function () { input1.value = input1.value; };

.onkeyup listens for things after you release the key from your keyboard, automatically firing a script after a change is made