using $view->preview() with two successive displays on a view gets the same output

  $view = views_get_view('my_view');
  // Get the output for first display.
  $view_html = $view->preview('display_1');
  // Get the output for 2nd display.
  $view_html = $view->preview('display_2');
  // $view_html is the same both times!!!

The problem is this line in execute():

    if (!empty($this->executed)) {
      return TRUE;
    }

Because the view is marked as having already been executed, the query for the second display isn't run.