Updating one Dropdownlist with another in Formview

<asp:FormView ID="FormView1" runat="server" AllowPaging="True" DataKeyNames="task_id"
        DataSourceID="SqlDataSource1" Style="z-index: 104; left: 12px; position: absolute;
        top: 190px" Width="610px" GridLines="Both" EmptyDataText="No data to display" OnItemDeleted="FormView1_ItemDeleted" OnItemDeleting="FormView1_ItemDeleting" OnItemInserted="FormView1_ItemInserted" 
         OnItemCreated="FormView1_ItemCreated" OnItemUpdated="FormView1_ItemUpdated" OnItemInserting="FormView1_ItemInserting" OnDataBound="FormView1_DataBound" ><PagerSettings FirstPageText="First" LastPageText="Last" Mode="NextPreviousFirstLast"
            NextPageText="Next" PreviousPageText="Previous" />
        <EditItemTemplate>
            <table style="z-index: 100; left: 3px; width: 600px; position: relative">
                <tr>
                    <td style="width: 110px; height: 25px">
                        Department:
                    </td>
                    <td style="width: 400px; height: 25px">
                        <asp:DropDownList ID="cboDepartments" runat="server" 
									DataSourceID="SqlDataSourceDept" AutoPostBack="true" onselectedindexchanged="cboDepartmentsE_SelectedIndexChanged" 
                            DataTextField="code_description" DataValueField="code_id" Style="z-index: 101;
                            position: relative; left: auto; top: 0px;" Width="220px" 
									SelectedValue='<%# Bind("department_id") %>' 
									>
                        </asp:DropDownList>
                    </td>
                </tr>
                <tr>
                    <td style="width: 110px; height: 25px">
                        Assigned to:
                    </td>
                    <td style="width: 400px; height: 25px">
                        <asp:DropDownList ID="cboAssignedTo" runat="server" DataSourceID="SqlDataSource6" DataTextField="USERNAME"
                            DataValueField="USER_ID" SelectedValue='<%# Bind("responsible_user_id") %>' Style="z-index: 105;
                            position: relative" Width="160px">
                        </asp:DropDownList>
                    </td>
                </tr>
            </table>
        </EditItemTemplate>
    </asp:FormView>
 
 
 
    <asp:SqlDataSource ID="SqlDataSource6" runat="server" ConnectionString="<%$ ConnectionStrings:Tempus %>"
        SelectCommand="SELECT 0 user_id,' None Specified' username from dual union select USER_ID, USERNAME FROM TMPT_USERS where end_date is null and (DEPARTMENT_ID = :DEPARTMENT_ID or DEPARTMENT_ID = 19)ORDER BY USERNAME"
        ProviderName="<%$ ConnectionStrings:Tempus.ProviderName %>">
        <SelectParameters>
            <asp:ControlParameter ControlID="ctl00$ContentPlaceHolder1$FormView1$cboDepartments" Name="DEPARTMENT_ID" PropertyName="SelectedValue" DefaultValue="0" />
       </SelectParameters>
     </asp:SqlDataSource>

                                  

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