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procedure TForm2.FormShow(Sender: TObject);
begin
  Form1.Hide;
end;

procedure TForm2.FormHide(Sender: TObject);
begin
  Form1.Show;
end;					

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...
  TForm1 = class(TForm)
    ApplicationEvents1: TApplicationEvents;
    procedure ApplicationEvents1Message(var Msg: tagMSG;
      var Handled: Boolean);
    procedure WMSysCommand(var Message: TWMSysCommand); message WM_SYSCOMMAND;
  private
    { Private-Deklarationen }
  public
    { Public-Deklarationen }
  end;
...
implementation
...
procedure TForm1.ApplicationEvents1Message(var Msg: tagMSG;
  var Handled: Boolean);
begin
 if (Msg.message = WM_SYSCOMMAND) and                // Minimize from taskbar
    (Msg.wParam and $FFF0 = SC_MINIMIZE) then begin
      Form2.ShowModal;
      Handled := True;
    end;
end;

procedure TForm1.WMSysCommand(var Message: TWMSysCommand);
begin
  with Message do                                   // Minimize from form
    if (CmdType and $FFF0 = SC_MINIMIZE) and (Application.MainForm = Self) then
      Form2.ShowModal
    else
      Inherited;
end;