<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns=false OnSelectedIndexChanged="GridView1_SelectedIndexChanged" OnSelectedIndexChanging="GridView1_SelectedIndexChanging">
        <Columns>
        <asp:TemplateField>
        <HeaderTemplate>
        DropDown 1
        </HeaderTemplate>
        <ItemTemplate>
        <asp:DropDownList ID="dd1" runat=server DataSource='<%#Load_DropDown()>' DataTextField="brand_name" DataValueField="brand_id"
        OnSelectedIndexChanged="Populate_Grid2" AutoPostBack=true> </asp:DropDownList>
        </ItemTemplate>
        </asp:TemplateField>
        
        <asp:TemplateField>
        <HeaderTemplate>
        DropDown 2
        </HeaderTemplate>
        <ItemTemplate>
        <asp:DropDownList ID="dd2" runat=server DataTextField="brand_name" DataValueField="brand_id"></asp:DropDownList>
        </ItemTemplate>
        </asp:TemplateField>        </Columns>
        </asp:GridView>

//function to determine which one is selected in dd1
 protected void Populate_Grid2(object sender, EventArgs e)
        {
         
         //locate the row in which the dropdown value has been changed
           GridViewRow gr=(GridViewRow)((DataControlFieldCell)((DropDownList)sender).Parent).Parent;
            //find the control in that
            DropDownList d1=(DropDownList) gr.FindControl("dd1");
            string selectedvalue=d1.selectedvalue;
       
            //using selectedvalue execute a query like 
            //select * from product where brand_id=selectevalue
            //get the result in datatable dt
            
            //located the second dropdown(dd2)
            DropDownList d2 = (DropDownList)gr.FindControl("dd2");
            d2.DataSource = dt;
            d2.DataBind();
            
        }