I just bought a 212x to mess with. I'd have to say, from about 300hz and up, there's
very little difference between a single 212x and two 112x cabs. Below 300hz, there's definitely more beef in the 212x. Here's my take..
If you're the kind of guy that likes a lot of beef onstage, or if the PA you're used to playing through isn't all that beefy, then I'd go with a 212x.
However if you're like me and you like your stage sound cleaner, then I'd go with the two 112x cabs. Keep in mind, you can't really judge a little cabinet by the typical listening position, which is usually about 6' away from the cab. If you take a listen down where the mic usually is, there's loads of beef there that the mic will pick up just fine. Let the mic pick all that up and then the PA will give you that girth & extension. Trust me, I've converted a handful of touring guitarists that I've worked with. They all thought they needed all that huge tubbyness onstage, but in all reality there was loads of it already in the mic. Everyone else in the band will love you as well because they're not having to listen through all that useless flab. Whether I'm doing FOH, monitors, or guitar teching, this is one trick that helps me get called back. However, there's always the bonehead that wants his pant legs to move from 20' away and doesn't give a shit about anyone else onstage. Luckily, that guy is usually the first to go.
Another benefit of the two smaller cabs is the ability to separate them, for a wider sound, or for monitoring reasons such as one cab behind you and one cabinet down in front angling up at your like a wedge.
Another cool trick is if you have a stereo tube power like a VHT, you can run one cab onstage on ch1 at a lower volume. And then run the 2nd cab offstage with ch2 opened up hotter, then mic the offstage cab. If you're playing a small room and don't have a good offstage area, you can put it in a foam lined trunk or under some wool blankets. I've gone as far as running a long speaker cord and a mic out to a cabinet that we left in the back of the truck.
For gigs with a bigger stage, I'm thinking I'll run the 212x as my dry center on ch1 of my VHT. And then use both 112x's as wide mono wet cabs on ch2 of the VHT. Haven't tried it yet but I have a feeling it'll be quite nice.