The crowd close to the stage will hear left/right effects, movement of sound, etc., and if you go to extremes there are guitars that have individual output for each string (as if that's needed), but you will never here true by definition stereo. A pair of mics on a cabinet is nothing new, and the sound can certainly be tailored to the players tastes, but that will usually entail micing with different mics placed in different locations relative to the cabinet drivers. I have played with front/back placement many, many times, and if you can get some good separation (not full left/right, but enough so that the presence of the guitar is "larger", you'll have a nice effect. 10 points for knowing what has to be done with the back mic. And this is for combo amps with open cabinets only.