A:

unit Unit1;

interface

uses
  SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls,
  Forms, Dialogs, Grids, Outline;

type
  TForm1 = class(TForm)
    Outline1: TOutline;
    Outline2: TOutline;
    procedure OutlineDragDrop(Sender, Source: TObject; X, Y: Integer);
    procedure OutlineMouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure OutlineDragOver(Sender, Source: TObject; X, Y: Integer;
      State: TDragState; var Accept: Boolean);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.OutlineDragDrop(Sender, Source: TObject; X, Y: Integer);
begin
  with Sender as TOutline do
    begin
      AddChild(GetItem(x,y),
          TOutline(Source).Items[TOutline(Source).SelectedItem].Text);
    end;

end;

procedure TForm1.OutlineMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  if Button = mbLeft then
    with Sender as TOutline do
      begin
        if GetItem(x,y) >= 0 then
          BeginDrag(False);
      end;
end;

procedure TForm1.OutlineDragOver(Sender, Source: TObject; X, Y: Integer;
  State: TDragState; var Accept: Boolean);
begin
  if (Source is TOutline) and (TOutline(Source).GetItem(x,y) <> TOutline(Source).SelectedItem) then
    Accept := True
  else
    Accept := False;
end;

end.