Impossible d'ajouter deux CollapsiblePanelExtender dans deux panel différents

<asp:Panel ID="p_MaxResultTitle" runat="server">
            <table class="header_table">
                <tr>
                    <td class="td_bordgauche">
                    </td>
                    <td class="td_tab0">
                        <asp:Image ID="Image1" runat="server" />
                        <asp:LinkButton ID="LinkButton1" runat="server" text="Options" CssClass="text_lb_white"></asp:LinkButton>
                    </td>
                    <td class="td_borddroit">
                    </td>
                    <td class="td_tab1">
                    </td>
                </tr>
            </table>

        </asp:Panel>
asp:Panel ID="p_MaxResult" runat="server" CssClass="panel_gridview">
            <asp:UpdatePanel ID="UpdatePanel1" runat="server">
                <ContentTemplate>
                    <table class="table_filtre">
                        <tr>
                            <td style="width:40%">
                                <asp:Label ID="State_Label" runat="server" Text="Nombre de résultat maximum par application:"></asp:Label>&nbsp;&nbsp;
                            </td>
                            <td><asp:TextBox ID="MaxElem_TextBox" runat="server" Columns="5">5</asp:TextBox></td>
                        </tr>
                    </table>

                </ContentTemplate>
            </asp:UpdatePanel>
        </asp:Panel>

        <asp:Panel ID="p_critereTitle" runat="server">
            <table class="header_table">
                <tr>
                    <td class="td_bordgauche">
                    </td>
                    <td class="td_tab0">
                        <asp:Image ID="Image2" runat="server" />
                        <asp:LinkButton ID="LinkButton2" runat="server" Text="Critères généraux" CssClass="text_lb_white"></asp:LinkButton>
                    </td>
                    <td class="td_borddroit">
                    </td>
                    <td class="td_tab1">
                    </td>
                </tr>
<%--                <asp:CollapsiblePanelExtender ID="CollapsiblePanelCritGle" runat="server" TargetControlID="p_criteres"
                    CollapsedSize="0" ExpandControlID="LinkButton2" CollapseControlID="LinkButton2"
                    AutoCollapse="False" AutoExpand="False" ExpandDirection="Vertical" SuppressPostBack="true"
                    ScrollContents="false" ImageControlID="Image2" ExpandedImage="../Images/Applidepliee.gif"
                    CollapsedImage="../Images/Applipliee.gif" />--%>

            </table>
        </asp:Panel>

<asp:Panel ID="p_criteres" runat="server" CssClass="panel_gridview">
            <asp:UpdatePanel ID="UpdatePanel2" runat="server">
                <ContentTemplate>
                    <table class="table_filtre">
......
......
                </ContentTemplate>
            </asp:UpdatePanel>
        </asp:Panel>

        private CollapsiblePanelExtender panelExtenderMaxResult = null;
        private CollapsiblePanelExtender panelExtenderCritRecherche = null;

protected void Page_Load(object sender, EventArgs e)
{
panelExtenderMaxResult = new CollapsiblePanelExtender();
            panelExtenderMaxResult.TargetControlID = "p_MaxResult";
            panelExtenderMaxResult.ExpandControlID = "LinkButton1";
            panelExtenderMaxResult.CollapseControlID = "LinkButton1";
            panelExtenderMaxResult.AutoCollapse = false;
            panelExtenderMaxResult.AutoExpand = false;
            panelExtenderMaxResult.ScrollContents = false;
            panelExtenderMaxResult.ExpandDirection = CollapsiblePanelExpandDirection.Vertical;
            panelExtenderMaxResult.SuppressPostBack = true;
            panelExtenderMaxResult.ImageControlID = "Image1";
            panelExtenderMaxResult.ExpandedImage = "../Images/Applidepliee.gif";
            panelExtenderMaxResult.CollapsedImage = "../Images/Applipliee.gif";

            panelExtenderCritRecherche = new CollapsiblePanelExtender();
            panelExtenderCritRecherche.TargetControlID = "p_criteres";
            panelExtenderCritRecherche.ExpandControlID = "LinkButton2";
            panelExtenderCritRecherche.CollapseControlID = "LinkButton2";
            panelExtenderCritRecherche.AutoCollapse = false;
            panelExtenderCritRecherche.AutoExpand = false;
            panelExtenderCritRecherche.ScrollContents = false;
            panelExtenderCritRecherche.ExpandDirection = CollapsiblePanelExpandDirection.Vertical;
            panelExtenderCritRecherche.SuppressPostBack = true;
            panelExtenderCritRecherche.ImageControlID = "Image2";
            panelExtenderCritRecherche.ExpandedImage = "../Images/Applidepliee.gif";
            panelExtenderCritRecherche.CollapsedImage = "../Images/Applipliee.gif";

if (!IsPostBack)
            {
                panelExtenderMaxResult.Collapsed = false;
                panelExtenderCritRecherche.Collapsed = false;

                p_MaxResultTitle.Controls.Add(panelExtenderMaxResult);

                p_critereTitle.Controls.Add(panelExtenderCritRecherche);

            }

}

p_MaxResultTitle.Controls.Add(panelExtenderMaxResult);

p_critereTitle.Controls.Add(panelExtenderCritRecherche);