The Valley of the Blue Snails, a really interesting blog mostly about an unusual setting that Canecorpus has created, has a post about Multi-Classing in his D&D setting:
I will be changing a few of the multi-class titles though I’m a bit mixed on what direction to take it. The titles are similar to normal class titles (Veteran, Cutpurse, Wizard, etc) in that they are mostly for fluff with perhaps a minor ability to adhere the two classes better. I’m deciding on wither to make it very setting specific or use more intuitive titles.
Example, a Fighter-Cleric would be a Paladin. Pretty intuitive. Setting specific would be something like a Dwarven Fighter-Cleric would be a Whitebeard. Not so intuitive but perhaps a better choice since this sort of multi-class fluff is well outside of the realm of B/X anyhow. The main problem is the setting specifics titles would indeed be rather specific, slanting towards race with specific classes.
I did something similar for a (for now abandoned) retro game I was working on, which I might as well share in case somebody finds it interesting:
Basically, there are six primary classes (one for each of the six standard stats) and they combine into 36 different classes, with differing emphasis depending on whether a particular class is primary or secondary. Somebody who’s primarily a Thief but uses magic to steal and con is a Mountebank, while somebody who is primarily a Mage, but uses stealth and deception to accomplish his ends and impress people with his power is a Charlatan, etc. You mostly got the armor restrictions of your primary class, and the weapon restrictions of your secondary class, with most other abilities splitting the difference. Spell user progressed as in their primary as if they were one level lower, and their secondary two levels lower. And so forth.
I actually think it’s pretty workable, but it’s not something my main face-to-face play group would be interested in, and I have too much on my plate right now to pursue it further. If I start a play-by-forum or play-by-post campaign, I’ll probably use Tunnels & Trolls instead of trying to sell people on and play-test some wacky homebrew.