Re: Is mixing 2 speakers from a 412, and a 212 cab possible?
That's an interesting question...one I never thought about. Physically and electronically, it is definitely possible. Some 412's have stereo inputs which isolate the speakers into two pairs. If yours has that arrangement, it's simple to try. Otherwise, it would need to be rewired, which isn't difficult. If your 212's have two parallel jacks each could be daisy chained to half the 412.
What it would sound like is another matter, but I'm guessing it would be fine since stereo 412's have two different signals going into them. I'm anxious to hear of the results if you try it. Just make sure you do proper impedance matching for best (and safe to the amp) results. For example, if the 412 has 4 16 ohms, each pair/half would be 8 ohms connected in parallel; then the 212's should have an overall impedance of 8 ohms as well; the amp would then see a 4 ohm load when half the 412 and the 212 were daisy chained in parallel.
:33: Very interesting!
EDIT: One other issue is speaker sensitivity. If those of the 412 are more than 3 db different than those of the 212's, there will be a noticeable volume difference.