<ajaxToolkit:PopupControlExtender ID="pce1" runat="server"
 TargetControlID="tbDeparture" PopupControlID="pnlCalendar" Position="Bottom" />
<ajaxToolkit:PopupControlExtender ID="pce2" runat="server"
 TargetControlID="tbReturn" PopupControlID="pnlCalendar" Position="Bottom" />