here you go this worked for me
<asp:TextBox runat="server" ID="txt1" OnTextChanged="txt1_TextChanged" AutoPostBack="true"></asp:TextBox>
<ajaxToolkit:CalendarExtender runat="server" ID="cal1" TargetControlID="txt1"></ajaxToolkit:CalendarExtender>
<asp:TextBox runat="server" ID="txt2"></asp:TextBox>
<ajaxToolkit:CalendarExtender runat="server" ID="cal2" TargetControlID="txt2"></ajaxToolkit:CalendarExtender>
protected void txt1_TextChanged(object sender, EventArgs e)
{
cal2.SelectedDate = Convert.ToDateTime(txt1.Text);
}
or you can do this through javascript I would recommend using jquery though to find the textboxes instead of using straight javascript
<asp:TextBox runat="server" ID="txt1" onchange="SetEndDate()"></asp:TextBox>
<ajaxToolkit:CalendarExtender runat="server" ID="cal1" TargetControlID="txt1"></ajaxToolkit:CalendarExtender>
<asp:TextBox runat="server" ID="txt2"></asp:TextBox>
<ajaxToolkit:CalendarExtender runat="server" ID="cal2" TargetControlID="txt2"></ajaxToolkit:CalendarExtender>
<script type="text/javascript">
function SetEndDate()
{
var txt1 = document.getElementById("txt1");
var txt2 = document.getElementById("txt2");
txt2.value = txt1.value
}
</script>