[delphi.com.ar]

Si mal no te entiendo, puedes utilizar lo que he comentado anteriormente de utilizar el Owner del form:

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Edit1: TEdit;
    Button2: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
  private
  public
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.Button1Click(Sender: TObject);
begin
  with TForm1.Create(Self) do
  try
    ShowModal;
  finally
    Free;
  end;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  if Owner is TForm Then
    TForm(Owner).Caption := Edit1.Text;
end;

end.