One grouse that I have with multi-classes and specializations is that there's very little lore to back it up. I always envison a multi-class as a truimphant achievement where the PC is able to take two very different disciplines and synergize them to achieve something unique.
Since PE is a new setting, for each multi-class, I hope there's at least some lore, device and/or NPC attached to it. It doesn't really make sense that the PC can multi-class without learning this somewhere via some means. Originally, someone must have founded an order by learning two separate disciplines before fusing them together. For an established multi-class, learning to multi-class could be easily achieved via learning from books or lore. An established guild or small chapter devoted to the multi-class would be nice.
For more exotic multi-class, it should only be allowed if the PC manages to achieve the objectives in a side-quest. A good example of this would be the Arcane Warrior for the mage in Dragon Age: Origins, where you learn the ancient secrets from the Elven Warriors/Mage via a memory gem.
Other esoteric multi-class could be introduced by NPCs or companions or via some other means. This is largely what was done is specializations in Dragon Age Origins.
Any thoughts?