Wrapping fields together in a Views Exposed Filter
I solved it by using a fieldset to group my fields.

  // Create a fieldset to group related fields.
  $form['my_group'] = array(
    '#type' =&gt; 'fieldset',
    '#attributes' =&gt; array(
      'class' =&gt; array('my-group')
    ),
  );

  // Move the desired fields in it.
  $form['my_group']['field_one'] = $form['field_one'];
  $form['my_group']['field_two'] = $form['field_two'];

  // Unset the original fields, so they don't show up in the form anymore.
  unset($form['field_one']);
  unset($form['field_two']);