The Multi-Classed Character: Oddly, this section doesn't provide any rules for how multi-classing at all. Instead it runs down all of the possible multi-class combinations (including the races that can access them), and gives a brief outline of each combo's strengths and weaknesses.
As established before, multi-class clerics can use edged weapons, and multi-class magic-users can cast spells in armour. Thieves are the only class that gets shafted by multi-classing, as their abilities can;t be used while wearing armour heavier than leather. This is all stuff that's been detailed earlier in the book, but it's nice to have it all in one place.
In OD&D, the only allowable multi-class combinations were fighter/thief, fighter/magic-user, fighter/magic-user/thief, and fighter/magic-user/cleric. All of these are available AD&D, and quite a few more exotic combinations are also included. Cleric/rangers sound cool. Half-orcs can opt to be cleric/thieves or cleric/assassins, which must say something about their religious beliefs. Gnomes can be fighter/illusionists or illusionist/thieves. I like the flexibility of multi-classing in 3rd edition, but there's a lot to be said for the distinctive flavour that comes with restricting multi-class combos by race.
The Character With Two Classes: Or dual-classing, as I know it, although that term isn't used here. Human characters whose stats are high enough can switch from one class to another, starting back at 1st level in the new class. They retain the hit points of the previous class, but none of the other abilities and powers. Those powers are still there, and they can be used, but if they are then the player in question gains no XP for that adventure. Only when the level of the new class exceeds that of the old can the character use the capabilities of his first class.
It's interesting to note that fighter/mages created using dual-classing aren't allowed to cast spells in armour. It seems that ability is restricted to multi-classed elves and half-elves, and that it's an ability restricted to them alone. Normally I'd scoff at any rule that makes elves more powerful, but I like this one. It's a good thematic fit.
Dual-classing was present in OD&D, but only in the vaguest sense. It's mentioned that characters with a high enough prime requisite for the new class may switch, but there are no hard rules given.