How can I bind 2 columns in one dropdownlist, one column below the other and acting as one column?

Question

I want my dropdown to consist of two columns that must act as a single column. The columns should be one below the other.

Like

wmsno12345

no12345

I've tried concatenating the columns but one column is coming in front of the other.

Binding for dropdown:

    void BindSalesOrder(string locationCode)
        {
            SOBL objSOBL = new SOBL();
            List<SalesHeaderEntity> lstSalesHeaderEntity = objSOBL.GetB2BSOList(locationCode);
            if (lstSalesHeaderEntity != null)
            {
                ddlB2BSaleOrder.DataSource = lstSalesHeaderEntity;
                if (Common.MyCInt(ViewState["DisplayOrderNoOnBatch"]) == 1)
                {
                    ddlB2BSaleOrder.DataTextField = "OrderNo";

                }
            else
            {
                ddlB2BSaleOrder.DataTextField = "No";

                }
            ddlB2BSaleOrder.DataValueField = "No";
                ddlB2BSaleOrder.DataBind();
                ddlB2BSaleOrder.Items.Insert(0, new ListItem("Please Select", "0"));
            }
            else
            {
                ddlB2BSaleOrder.Items.Clear();
                ddlB2BSaleOrder.Items.Insert(0, new ListItem("Please Select", "0"));
            }
        }

Dropdown

 <td class="col-md-3 form-group">
       <asp:DropDownList ID="ddlB2BSaleOrder" runat="server" CssClass="form-control js-example-placeholder-single" AutoPostBack="true"
                                            OnSelectedIndexChanged="ddlB2BSaleOrder_SelectedIndexChanged" ToolTip="Select ">
            </asp:DropDownList>
            <asp:RequiredFieldValidator SetFocusOnError="true" ID="rfvB2BSaleOrder" CssClass="Asterix"
                                            Visible="false" InitialValue="0" ValidationGroup="Save" runat="server" ControlToValidate="ddlB2BSaleOrder"
                                            ErrorMessage="Please select Sale Order." Display="None">*</asp:RequiredFieldValidator>
      </td>