type
  TForm1 = class(TForm)
  {...}
  private
    { Private-Deklarationen }
    procedure OnAppMessage(var Msg: TMsg; var Handled: Boolean);
  {...}
  end;

{--------------------------------------------------------------------------------------------------}

procedure TForm1.FormCreate(Sender: TObject);
begin
  Application.OnMessage := OnAppMessage;
end;

{--------------------------------------------------------------------------------------------------}

procedure TForm1.OnAppMessage(var Msg: TMsg; var Handled: Boolean);
begin
  Handled := False;
  if Msg.message = WM_KEYDOWN then
    if Msg.wParam = VK_TAB then
      if (ActiveControl = Edit1) or (ActiveControl = Edit2) then
        Handled := True;
end;

{--------------------------------------------------------------------------------------------------}