Dual class is restricted to humans and is when an exceptional PC (prime requisites of old class 15+, new class 17+) decides to change his class later on in his career. As he has to concentrate to learn new things, he cannot use his old abilities without XP penalties until his new level surpasses his old level. The only thing that he retains until then are his old HP (he keeps his old HP and does not gain any more until he surpasses his old level).
Multi-classing is what demi-humans do, as they have the breadth and length of life that is needed to do several things simultaneously.
In their case, HP are sort of averaged out by class.
(If I am wrong about the ability requirements, sorry...)