How to handle nested ModalpopupExtender

Hi:

I meet a problem about how to handle nested modalpopupextender, 

My code snippet is as following:

<asp:Button ID="btnDummy3" runat="server" Style="visibility: hidden" /><cc3:ModalPopupExtender ID="modalPopupExtender1" runat="server"
            BehaviorID="modalPopupExtender1"
            TargetControlID="btnDummy3"
            PopupControlID="uppnlPreViewPDF"
            BackgroundCssClass="modalBackground"></cc3:ModalPopupExtender><asp:Button ID="btnDummy5" runat="server" Style="visibility: hidden" /><cc3:ModalPopupExtender ID="ModalPopupExtender2" runat="server"
            BehaviorID="modalPopupExtender2"
            TargetControlID="btnDummy5"
            PopupControlID="uppnlHandlePDF"
            BackgroundCssClass="modalBackground"></cc3:ModalPopupExtender><asp:UpdatePanel ID="uppnlPreViewPDF" runat="server" ChildrenAsTriggers="true"><ContentTemplate><asp:Panel ID="Panel1" runat="server" Width="100%">
                ....</asp:Panel></ContentTemplate></asp:UpdatePanel><asp:UpdatePanel ID="uppnlHandlePDF" runat="server" ChildrenAsTriggers="true"><ContentTemplate><div runat="server" class="container" style="position: relative; z-index: 900001 !important;"><asp:Panel ID="pnlMakeUpPDF" class="form-horizontal" role="form" runat="server" BackColor="#F2F2DC" Width="385px" Height="270px">
                 ......</asp:Panel></div></ContentTemplate></asp:UpdatePanel>

The problem is how to call these two modalpopupextender and let then show in the same time, 

I mean if I call modalPopupExtender1.show() and the call modalPopupExtender2.show(), the UI should show modalPopupExtender1

and modalPopupExtender2 below the modalPopupExtender1. 

But in fact it always only show either modalPopupExtender1 or modalPopupExtender2, even I have set z-index, 

it still not working. Is there any good method to handle that, thanks a lot.

By the way, in chrome, it indeed can show nested modalpopextenders but in IE11, it fail.

Any suggestion is highly appreciated, thanks.