Given that this is the coding forum, you're probably not really after what I have to say, but... Just ignore me if I'm way off what you want. And I'm a builder, so I can't tell you how to CODE any of this.
My experience with multiclassing in the past has been pretty much the way 2nd edition AD&D handled it, which was you picked out two or three classes at creation, and advanced simultaneously. Of course, given a max level of x, if multiclasses can advance to x in each class, that leads to some rather severe munchkinness. So the part of me that's been an admin and a AD&D DM for the last 5 years says no, any system that allows you to get that kinda power is a bad, bad thing.
Dwip's First Law of Multiclassing: If you have X levels, multiclasses should max at X like normal classes.
Having blathered all that, I'd set it up something like this.
The preferable way to do it is sort of like how 3e D&D or Neverwinter Nights does it. You start out as class A, and at every level, you can chose to pick up another class. This obviously isn't going to work very well if you don't have a 3e style unified xp chart for all the classes.
The other option is the old at creation, you pick two or three or something classes, and that's it. Lots easier to deal with if you don't have ye olde unified xp chart.
In the first case, you don't need to have any kind of expratio command or anything, and in the second, it's probably prefferable to do something more like the classexp1/2 commands just to slow down multis so they don't become ubermunchkins of doom.
Dunno if that's what you were after, but there you go.