Problems with teaser when WYSIWYG module is enabled

When testing various WYSIWYG solutions it may happen that WYSIWYG and CKEditor module are enabled at the same time.
It may even happen that someone have two modules installed intentionally because he wants to use two different wysiwygs on different textareas.

The problem: WYSIWYG module removes information about teaser.
This is caused by:

function wysiwyg_form_alter(&$form, &$form_state) {
  $form['#after_build'][] = 'wysiwyg_process_form';
  // Teaser splitter is unconditionally removed and NOT supported.
  if (isset($form['body_field'])) {
    unset($form['body_field']['teaser_js']);
  }
}

in wysiwyg.module

The problem and possible workaround is similar as in #666560: Compatibility with Resizable Body.