$mysql->query("SELECT forum_topics.id AS `forum_topic_id`,
forum_topics.forum_id,
forum_topics.forum_cat_id,
forum_topics.topic,
forum_topics.author,
forum_topics.date,
forum_topics.last_post_uid,
forum_topics.last_post_date,
users.username
FROM `forum_topics`
LEFT JOIN `users` as u1 ON u1.id = forum_topics.author
LEFT JOIN `users` as u2 ON u2.id = forum_topics.last_post_uid
WHERE `forum_id`='$id_forum'");

una cosa come questa dovrebbe andare, anche se il secondo ON non sono sicuro di aver beccato il campo giusto