두개의 실행모듈간 interface 가 필요합니다.
팁에 게시된 오병종님의 Source 를 참조했으나 잘안되어서.....
Delphi V4 이고요 os는 Windows me 입니다.......

Sned.exe Source
-----------------------------------------
unit Send;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms,
  WinTypes, WinProcs, StdCtrls, Grids, ExtCtrls, Buttons, shellapi;

type
  TsendF = class(TForm)
    Label1: TLabel;
    Button1: TButton;
    Memo1: TMemo;
    Button4: TButton;
    procedure Button1Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure Button4Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  sendF: TsendF;

implementation

{$R *.DFM}

procedure TsendF.Button1Click(Sender: TObject);
var
   hWndMe  : HWnd;
   str1,str2 : string;
begin
   str1 := '12345';
   hwndme := FindWindow('TApplication', pchar(str1));
   if (hWndMe <> 0) then
      begin
      showmessage(' 이미 수행중입니다.!!');
      SetactiveWindow(hWndMe);
      BringWindowToTop(hWndMe);
      ShowWindow(hWndMe, SW_SHOWNORMAL);
      exit;
      end;

   str2 := 'project2.exe ' + '12345 ';
   shellExecute(0,'',PChar(ExtractFileDirApplication.ExeName) +
               '\Project2.exe'), PChar(str2),'',SW_SHOWNORMAL);
end;

procedure TsendF.FormCreate(Sender: TObject);
var
   hWndMe  : HWnd;
begin

   hwndme := FindWindow('TApplication','send');
   if (hWndMe <> 0) then
      begin
      showmessage(' 이미 send 프로그램이 수행중입니다.!!');
      SetForegroundWindow(hWndMe);
      SetactiveWindow(hWndMe);
      BringWindowToTop(hWndMe);
      ShowWindow(hWndMe, SW_SHOWNORMAL);
      exit;
      end
   else
      begin
      Application.Title := 'send';
      hwndme := FindWindow('TApplication','send');
      BringWindowToTop(hWndMe);
      end;

end;

procedure TsendF.Button4Click(Sender: TObject);
var  hWndMe  : HWnd;
     str1 : string;
begin
   str1 := '12345';
   hwndme := FindWindow('TApplication', pchar(str1));
   if (hWndMe <> 0) then
      begin
      SendMessage(hwndme, WM_USER + 1,1,1 );
//      SendMessage(hwndme, WM_CLOSE,0,0 );
// WM_CLOSE 는 잘됨.....
      end;
end;

Receive.exe Source
-----------------------------------------
unit Receive;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms,
  Dialogs, WinTypes, WinProcs, StdCtrls, Grids, ExtCtrls, Buttons,
  shellapi;

type
  TReceiveF = class(TForm)
    Button1: TButton;
    Memo1: TMemo;
    Label1: TLabel;
    procedure FormCreate(Sender: TObject);
    procedure UserFunction(var AUserMessage: TMessage); Message
                          WM_USER + 1;
  private
    { Private declarations }
  public
    { Public declarations }
end;

var
  ReceiveF: TReceiveF;

implementation

{$R *.DFM}

procedure TReceiveF.UserFunction(var AUserMessage: TMessage);
var
begin
     showmessage('receive');
end;

procedure TReceiveF.FormCreate(Sender: TObject);
var
   hWndMe  : HWnd;
   d_strTmp : string;
begin
   d_strTmp :=  paramstr(2);
   hwndme := FindWindow('TApplication',pchar(d_strTmp));
   if (hWndMe <> 0) then
      begin
      showmessage(' 이미 rec 프로그램이 수행중입니다.!!');
      SetForegroundWindow(hWndMe);
      SetactiveWindow(hWndMe);
      BringWindowToTop(hWndMe);
      ShowWindow(hWndMe, SW_SHOWNORMAL);
      exit;
      end
   else
      begin
      Application.Title := d_strTmp;
      hwndme := FindWindow('TApplication',pchar(d_strTmp));
      BringWindowToTop(hWndMe);
      end;

end;

end.end.