<asp:LinkButton ID="link331"  text="View`1"   runat="server" OnClick="Lnk31_Click"/>
<asp:LinkButton ID="link332"  text="View`2"   runat="server" OnClick="Lnk31_Click"/>

protected void Lnk31_Click(object sender, EventArgs e)
    {
      
        switch ((sender as LinkButton).ID)
        {
            case "link331":
                Session["Target"] = "link331";
                mltv1.ActiveViewIndex = 0;
                ModalPopupExtender1.Show();
                break;
            case "link332":
                Session["Target"] = "link332";
                mltv1.ActiveViewIndex = 1;
                ModalPopupExtender1.Show();
                break;
            default:
                break;
        }
    }

 <asp:Panel ID="Panel1" runat="server" CssClass="popup" Style="display: none">
            
            <asp:UpdatePanel ID="upd1" runat="server" UpdateMode="Conditional">
                <ContentTemplate>
                    <asp:MultiView ID="mltv1" runat="server" OnPreRender="mltv1_PreRender">
                        <asp:View ID="View1" runat="server">
                           
                            <asp:Label ID="lb1" Text="1" runat="server"></asp:Label>
                           
                          <asp:Button ID="Button1" runat="server" Text="Начать" CssClass="button" OnClick="Button1_Click" />
                           <%-- <asp:Button ID="Button22" Text="Закрыть" runat="server" CssClass="button" />--%>
                        </asp:View>
                        <asp:View ID="View2" runat="server">
                           
                            <asp:Label ID="lb2" Text="2" runat="server"></asp:Label>
                            <asp:Button ID="Button5" runat="server" Text="Вернуться" CssClass="button" OnClick="Button11_Click" />
                          <%-- <asp:Button ID="Button2" Text="Закрыть" runat="server" CssClass="button" />--%>
                       </asp:View>
                    </asp:MultiView>
                  <asp:Button ID="Button2" Text="ЗАКРЫТЬ!!!" runat="server" CssClass="button" />

                </ContentTemplate>
            </asp:UpdatePanel>
        </asp:Panel>
    
                   
    <ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender1" runat="server"
      PopupControlID="Panel1" TargetControlID="link332" 
      BackgroundCssClass="content" OnPreRender="ModalPopupExtender1_PreRender" CancelControlID="Button2" />  

 protected void mpe1_Close(object sender, EventArgs e)
    {
     ModalPopupExtender1.Hide();
    }