You can do it by using JavaScript :-
I. Write a Javascript function that will set your first textbox value to second textbox.
Eg.
function CopyText() {
var FirstTextBox = $get('<%= txtBox1.ClientID %>');
var SecondTextBox = $get('<%= txtBox2.ClientID %>');
SecondTextBox.value = FirstTextBox.value;
}
II. Now take two textbox and in first textbox, use onkeyup event to call that javascript function.
Eg.
<asp:textbox id="txtBox1" runat="server" onkeyup="CopyText()"></asp:textbox>
<br />
<asp:textbox id="txtBox2" runat="server"></asp:textbox>
I hope, now you are clear.
Gool luck.