multiple use of attachment_fields_to_save

Hello,

Can the filter attachment_fields_to_save be used in more than one plugin simultaneously? I have one plugin using it which is working, and I tried to add another but the second instance of it is not saving the fields (though it is showing them). The code in the second plugin is, in brief:

function __construct() {
  //... some other stuff
  add_filter( 'attachment_fields_to_save', array( &$this, 'attachment_save_post'), 10, 2 );
  }

  function attachment_save_post($post, $attachment) {
        error_log("in attachment save post");
    $this->save_post($post['ID'], $attachment);
  }

The attachment_save_post function is not getting hit – the error log line is not appearing in the error log.