As noted by someone upthread, the salient question is "why do Humans and non-Humans use different systems?"; a question to which I've never seen or heard a sensible answer.
A corollary question, particularly as Gygax favoured a Human-centric game, is "if he wanted to promote Humans, why did he make the Human system more restrictive than the non-Human system?"
Also, if a character has all 18s for stats and thus meets every requirement for everything, is there anywhere a written rule that says a dual-classed Human cannot pick up a third class after the second class' level has caught up to the first? I don't recall ever seeing one, though as I ditched dual-classing ages ago it's a long time since I paid much attention to the minutae of it. And Elves can potentially triple-class, so the game does have precedent for three-classed characters.