static class Program
{
  [STAThread]
  static void Main()
  {
    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    Form1 splashForm = null;
    var splashThread = new Thread(() =>
    {
      splashForm = new Form1(); // сплэш-форма
      Application.Run(splashForm);
    });
    splashThread.SetApartmentState(ApartmentState.STA);
    splashThread.Start();
    var mainForm = new Form2(); // главная форма.
    mainForm.Load += (s, e) => splashForm.BeginInvoke((Action) (() => splashForm.Close()));
    Application.Run(mainForm);
  }
}