You try this code its work...
Using javascript in aspx page
<asp:TextBox ID="TextBox1" runat="server" onkeyup="document.getElementById('TextBox2').value = document.getElementById('TextBox1').value"></asp:TextBox>
<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
or
Enable Autopostback property is true then write this code in codebehind
TextChageEevent
textbox2.text=textbox1.text;
Happy coding....