[WkaymQ48]

Hola,

yo lo haría así:

Código Delphi [-]
procedure UnirArchivos( nFile1, nFile2, nFileOut: string );
var
  File1,File2,FileOut: TFileStream;
begin
  File1 := TFileStream.Create(nFile1,fmOpenRead);
  try
    File2 := TFileStream.Create(nFile1,fmOpenRead);
    try
      FileOut := TFileStream.Create(nFile1,fmCreate);
      try
        FileOut.CopyFrom(File1,0);
        FileOut.CopyFrom(File2,0);
      finally
        FileOut.Free;        
      end;
    finally
      File2.Free;
    end;
  finally
    File1.Free;
  end;
end;

pero si nos ajustamos a lo que pides:

Código Delphi [-]
procedure UnirArchivos( nFile1, nFile2, nFileOut: string );
var
  File1,File2,FileOut: TMemoryStream;
begin
  File1 := TMemoryStream.Create;
  File2 := TMemoryStream.Create;
  FileOut := TMemoryStream.Create;
  File1.LoadFromFile(nFile1);
  File2.LoadFromFile(nFile2);

  FileOut.CopyFrom(File1,0);
  FileOut.CopyFrom(File2,0);

  FileMpo.SaveToFile(nFileMPO);

  FileL.Free;
  FileR.Free;
  FileMPO.Free;
end;

¿Estas creando imágenes mpo?