you can use JavaScript, here is the example:-
your TextBox:-
<asp:textbox id="txtbox1" runat="server" onkeyup="changeText()"></asp:textbox>
<asp:textbox id="txtBox2" runat="server" ></asp:textbox>
here is the Javascript function, which has been called on onkeyup event of txtbox1:
function changeText() {
var txt1 = document.getElementById("<%= txtbox1.ClientID %>");
var txt2 = document.getElementById("<%= txtBox2.ClientID %>");
txt2.value = txt1.value;
}
Good luck.