Try to use onkeyup
ASP Code
<asp:textbox id="TextBox1" onkeyup="show()" runat="server" xmlns:asp="#unknown"></asp:textbox>
<asp:textbox id="TextBox2" runat="server" xmlns:asp="#unknown"></asp:textbox>
Javascript Code
<script type="text/javascript">
function show() {
var txt1 = document.getElementById("TextBox1").value;
document.getElementById("TextBox2").value = txt1;
}
</script>
I hope my idea is right. :)