The quickest and easiest way would be to use jQuery.
Include the library:
<script src="http://code.jquery.com/jquery-latest.min.js"
type="text/javascript"></script>
HTML:
<input type="text" id="textOne" onkeyup="clone();" />
<input type="text" id="textTwo" />
Javascript:
function clone() {
$("#textTwo").val($("#textOne").val());
}
Here is a demo: http://jsfiddle.net/5c8a9w39/
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solved How to clone input text into the other input text using Javascript?