When two or more MaskInput controls are focused at the same time, the application freezes.

Workaround:

Instead of 

  txtMask1.Focus();
  txtMask2.Focus();

Use

  txtMask1.Focus();
                Dispatcher.BeginInvoke(new Action(() =>
                    {
                        txtMask2.Focus();
                    }), DispatcherPriority.Loaded);