Fusionner une cellule dans un gridView

<asp:gridview id="gridview1" runat="server" allowsorting="true" autogeneratecolumns="false"
                    backcolor="white" bordercolor="#dedfde" borderstyle="none" borderwidth="1px"
                    cellpadding="5" datasourceid="dseventsdoc" forecolor="black" gridlines="vertical"
                    height="0px" width="850px" datakeynames="pk_document" cssclass="style3" ondatabinding="gridview1_databinding"
                    ondatabound="gridview1_databound" onload="gridview1_load">
                    <alternatingrowstyle backcolor="white" />
                    <columns>
                        <asp:boundfield datafield="designation_document" headertext="designation_document"
                            sortexpression="designation_document" readonly="true" />
                        <asp:boundfield datafield="designation_type" headertext="designation_type" sortexpression="designation_type"
                            readonly="true" />
                        <asp:boundfield datafield="date" headertext="date" sortexpression="date" dataformatstring="{0:dd/mm/yyyy}"
                            readonly="true" />
                        <asp:templatefield headertext="commentaire" sortexpression="commentaire">
                            <edititemtemplate>
                                <asp:textbox id="textbox1" runat="server" height="81px" text='<%# bind("commentaire") %>'
                                    width="264px"></asp:textbox>
                            </edititemtemplate>
                            <itemtemplate>
                                <asp:label id="label1" runat="server" text='<%# bind("commentaire") %>'></asp:label></itemtemplate>
                        </asp:templatefield>
                        <asp:templatefield showheader="false">
                            <footertemplate>
                                <asp:hyperlink id="modifier_supprimer" runat="server">modifier/supprimer</asp:hyperlink></footertemplate>
                            <itemtemplate>
                                <asp:button id="buttonouvrir" runat="server" causesvalidation="true" commandname='<%# bind("designation_document")%>'
                                    commandargument='<%# bind("emplacement_type") %>' text="ouvrir" onclick="buttonouvrir_click" /></itemtemplate>
                        </asp:templatefield>
                        <asp:commandfield showeditbutton="true" />
                        <asp:templatefield>
                            <itemtemplate>
                                <asp:linkbutton id="liencomposantslieraudocument" runat="server" commandargument='<%# bind("pk_document") %>'
                                    text="composants liés au document" onclick="liencomposantslieraudocument_click"></asp:linkbutton></itemtemplate>
                        </asp:templatefield>
                        <asp:templatefield showheader="false">
                            <itemtemplate>
                                <asp:linkbutton id="linksupprimerdoc" runat="server" causesvalidation="true" onclientclick="return confirm_delete()"
                                    text="supprimer" commandargument='<%# bind("pk_document")%>' onclick="linksupprimerdoc_click"></asp:linkbutton></itemtemplate>
                        </asp:templatefield>
                    </columns>
                    <footerstyle backcolor="#cccc99" />
                    <headerstyle backcolor="#6b696b" font-bold="true" forecolor="white" />
                    <pagerstyle backcolor="#f7f7de" forecolor="black" horizontalalign="right" />
                    <rowstyle backcolor="#f7f7de" />
                    <selectedrowstyle backcolor="#ce5d5a" font-bold="true" forecolor="white" />
                </asp:gridview>