I've found this on another side (which is german) and wondered, why this isn't posted here. I translated as good as possible. On this site it's confirmed working.
You just need 2 accounts.
Char A and Char B group together.
Both enter the dungeon.
Char A clears the dungeon, Char B is waiting at the entrance.
Char A logs out/off (don't know what is correct English).
Char B is alone. He leaves the dungeon and resets.
Char A logs in again. He is automatically in a new ID.
Regroup again and do it again.
This exploit is extremely helpful if you are farming the Baron Mount in Stratholme. Enjoy! :wave: