One weird thing I always thought about the whole dual-class thing, though: half-elves were included as being able to reach unlimited levels in 1e in the bard class, but half-elves cannot dual-class!
Thus, I guess that maybe it was intended for half-elves to be able to dual-class only if they intended to be bards in the future? (This is what I would guess, but a fun way would be to allow bards to start as a multi-classed fighter/thief/bard instead
).
In 2e, the Forgotten Realms had a weird underwater class called the dukar, which was available to any race as a dual-classing option (so maybe DC should keep the "option" available, just in case).