It is not possible to do on purpose. It did happen to me by accident though. I posted a thread in #offtopic asking for feedback on my first Photoshop project I did -- so I put the #feedback in the tags. The thread got a fair amount of replies but eventually died down. Then the Destiny beta came out and I started to get replies to my thread again. During this time, Bungie created the #feedback Forms and my thread was in both #offtopic and #feedback. But it was primarily in the #feedback. If you were to click on the topic in #offtopic, it would move you to #feedback. You could tell because the "Feedback" banner was at the top of the thread. And if you were to reply to it, it would bump the thread in #feedback but not #offtopic.