I am in the process of trying to do what your asking about right now.....and I actually own a very nice 4X12 cab still.
There are certain things I love about the 4X12, and certain things I dont. Ive got one of those Krankenstein cabs loaded with texas heats, which is a VERY well built HEAVY cab. I very much dislike dragging this thing to the practice room and smaller shows, so I decided I should try a single 2X12 cab. I can keep the little one at home, the big one at the rehearsal space, and for smaller shows I only gotta move the 2X12.
I did not expect great things from the 2X12.....but now Im finding that I do like it, and there are lots of options. I bought a closed back "AVATAR" 2X12 with celestion V30's and I am liking these speakers more than anything with a recto head(so far). The upper mids in these cabs really help bring out certain things in the recto that usually arent there.
I liked this 2X12 enough that I started looking around at others too. There are other closed backs, ported, openbacks, different dimension cabs, speakers and everything at your disposal with 2 different cabs!
If you buy a 4X12....youve got a 4X12. Two sepearate 2X12's have more "mix and match" options(not just talking speakers here.....but ports, no ports, open/closed, different dimensions between the two cabs and everything can be changed around to tweak your sound). Two seperate 2X12's done right can cover more "tonal ground" than your basic 4X12 even with swapping around speakers.