This will most certainly be very difficult to balance. My Paladin Priest will have infinite healing :]
I've always played a Paladin as a main character and every time I try to make it a hybrid between Tank and DPS, allocating some skills for damage dealing, sacrificing some defense, but that's all.
I understand that it is difficult and perhaps it should have been raised at another point in development. But if they ever want to do it, they could use a system similar to d&d: there is a total level of classes, so each level in one class implies one less level in another class, this implies that you will not get high skills in any class, which justifies exploiting the combination of low-level skills. If there is still a particular skill that breaks the balance, it will be prohibited from acquiring it in multiclass