Revert to previous dropdown options after change

You’re setting the HTML of the second drop down to essentially blank with:

$('select[name="second_dropwdown"]').html("<option>Sorry, no options</option>");

So when you enable it again, that’s the only option. Instead, you might try including that as an actual option within your select and then selecting it when disabling the second drop down. You could either include it in your HTML or add it via JavaScript. Here’s an example of the latter.

  jQuery(document).ready(function ($) {
      var $secondDropdown = $('#second_dropdown'), 
          $noOptions;

      // adding the the option with js
      $secondDropdown.append('<option id="no-options">Sorry, no options!</option>');

      // by default, no one needs to see it.
      $noOptions = $('#no-options').hide();

      $('select[name="post_type"]').change(function (event) {
          if ($('select[name="post_type"]').val() === 'test') {
              // Now, select the option and show it
              $noOptions.prop('selected', true).show();
              $secondDropdown.prop('disabled', true).trigger('chosen:updated');
          } else {
              // back in business so let's hide it again.
              $noOptions.hide();
              $secondDropdown.prop('disabled', false).trigger('chosen:updated');
          }
      });
  });

Here’s a live example: http://jsfiddle.net/4ngmhmup/