Here is a great cheat as to how you can actually duel inside of the north bank at dalaran.
1 – Find the duel flags that are shown in the picture above. A hint will be they are at the ne ramp right outside of the circle of will. This is important because you have to be inside of the circle of wills dueling area and not in the underbelly
2 – Now mount up and have the person you want to duel mount up as well. Run along the red line shown in the picture above. The line will take you right to the ramp and you will get a message to pop up. The message will say that you are going to get out of dueling mode in 10 seconds from traveling to far. Keep on going up the ramp instead of turning around shown in the picture below
3 – Now go to the well in the picture above and start dueling to everyone’s amazement.