unit Unit1;
interface
uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;
type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
  public
  end;
var
   Form1: TForm1;

implementation
uses Unit2;

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
   Form2.ShowModal;
end;

end.

unit Unit2;
interface
uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs;
type
  TForm2 = class(TForm)
  private
    procedure WMSYSCOMMAND(var Msg: TWMSYSCOMMAND); message WM_SYSCOMMAND;
    procedure WMACTIVATEAPP(var Msg: TWMACTIVATEAPP);
  public
  end;
var
  Form2: TForm2;

implementation
{$R *.dfm}

procedure TForm2.WMSYSCOMMAND(var Msg: TWMSYSCOMMAND);
begin
   if (Application.MainForm.Handle>0) and (Msg.CmdType=SC_MINIMIZE) then
      ShowWindow(Application.MainForm.Handle, SW_MINIMIZE)
   else
      inherited;
end;

procedure TForm2.WMACTIVATEAPP(var Msg: TWMACTIVATEAPP);
begin
   if Application.MainForm.Handle>0 then begin
      if IsIconic(Application.MainForm.Handle) then
         ShowWindow(Application.MainForm.Handle, SW_RESTORE);
   end;
   inherited;
end;

end.

UNIT Unit3;
INTERFACE
USES
   Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
   Vcl.Controls, Vcl.Forms, Vcl.Dialogs;
type
   TForm3 = class(TForm)
   private
      procedure WMSYSCOMMAND(var Msg: TWMSYSCOMMAND); message WM_SYSCOMMAND;
      procedure WMACTIVATEAPP(var Msg: TWMACTIVATEAPP); message WM_ACTIVATEAPP;
   public
   end;
var
  Form3: TForm3;

IMPLEMENTATION
{$R *.dfm}
uses Unit1;

procedure TForm3.WMSYSCOMMAND(var Msg: TWMSYSCOMMAND);
begin
   if (Application.MainForm.Handle>0) and (Msg.CmdType=SC_MINIMIZE) then begin
      ShowWindow(Form1.Handle, SW_SHOW);
      ShowWindow(Application.MainForm.Handle, SW_MINIMIZE);
   end;
   inherited;
end;

procedure TForm3.WMACTIVATEAPP(var Msg: TWMACTIVATEAPP);
begin
   if IsIconic(Application.MainForm.Handle) then begin
      ShowWindow(Form1.Handle, SW_HIDE);
      ShowWindow(Application.MainForm.Handle, SW_RESTORE);
   end;
   inherited;
end;

END.