[[escafandra]]

No me di cuenta que usabas Lazarus. Te paso el código que he compilado en Lazarus pero probado con un solo monitor, no dispongo de configuración con dos minitores:

Código Delphi [-]
uses   Windows, ....;  
function EnumDisplayMonitors(dc: HDC; lprcClip: PRECT; lpfnEnum: pointer; dwData: LPARAM): boolean; stdcall; external 'User32.dll' name 'EnumDisplayMonitors';  

implementation   
{$R *.lfm}    

function MonitorEnumProc(hMonitor: THANDLE; hdcMonitor: HDC; var lprcMonitor: RECT; dwData: LPARAM): boolean; stdcall;
{$J+}
const
   n: integer = 0; 
var   
  W, H: integer;   
  DC: HDC;   
  bmp, oldbmp: HBITMAP;   
  Bitmap: TBitmap; 
begin
  W:= lprcMonitor.Right - lprcMonitor.Left;   
  H:= lprcMonitor.Bottom - lprcMonitor.Top;
  DC:= CreateCompatibleDC(0);
  bmp:= CreateCompatibleBitmap(hdcMonitor, W, H); 
  oldbmp:= SelectObject(DC, bmp); 
  BitBlt(DC, 0, 0, W, H, hdcMonitor, lprcMonitor.Left, lprcMonitor.Top, SRCCOPY + $40000000);   SelectObject(DC, oldbmp);
  DeleteObject(DC); 
  Bitmap:= TBitmap.Create; 
  Bitmap.Handle:= bmp; 
  Bitmap.SaveToFile(Format('panta%d.bmp', [n])); 
  inc(n); 
  Bitmap.Free; 
  Result:= true; 
{$J-} 
end;  

procedure CaptureMonitors; 
var
  VirtualScreenDC: HDC; 
begin 
  VirtualScreenDC:= GetDC(0); 
  EnumDisplayMonitors(VirtualScreenDC, nil, @MonitorEnumProc, 0); 
  ReleaseDC(0, VirtualScreenDC); 
end;     

{ TForm1 }  

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

Espero que te sirva de ayuda.

Saludos.