A dual class system could be done two ways, some class combo would have mutually exclusive talent, if you pick mage pally combo, you would have to chose between bubble or IB, of have both with a DR on the second activation.
Or we could get a more boring dual class implementation, just allowing you to re class on the fly, like specs, but with classes.
Yes this would probably be a little imbalanced and a rocky road from thebget go, but it would be so much fun.