Hey there.
Before you start reading, to get 2 QD weapons, you need to already have one + use the faction change. I apologize for my english that isn't that really good.
I finished the quest for Quel'Dalar 2 days ago. Yesterday I decided to used the change faction fonction ( Alliance to Horde), and than I noticed that I had a quest in the horde inn of Dalaran. Oh, great. Then I realized that it was a part of the quest line of Q'D.
[The fact is that when you become a member of the opposite faction, you can do almost every quest in northrend or outland, even if you did them previously. I mean, you can do the horde's quests in a zone, even if you completed them previously as an alliance member.]
So i took the quest, without any conviction that I could use 2 QD weapons at the end, and I did it.
http://bit.ly/6f9EM9
3 instances later and a travel to Sunwell...
http://bit.ly/8Xdwq3
I finished the questline. And, finally... I could take another weapon, since it's not unique
http://bit.ly/5Cq3u6
http://bit.ly/87AxT0
So... Before using the faction change, make sure you've completed the questline, so you can get another
Edit: added link