Use
<asp:panel id="repeter_panel" runat="server" cssclass="modalPopup" style="display:none;" xmlns:asp="#unknown"> </asp:panel>
instead of
<asp:panel id="repeter_panel" runat="server" cssclass="modalPopup" visible="False" xmlns:asp="#unknown"></asp:panel>
and i can use as many modalpopupextender.
Thank you.