H2, with any _coincident, two-microphone_ stereo recording you can sum the channels to derive an "M" signal and take their difference to derive an "S" signal, then equalize M and S separately if you want (I often boost the bass in the "S" channel), and recombine them in any desired proportion, and there shouldn't be much destructive interference between channels to speak of.
With spaced-microphone recordings you could derive "M" and "S" signals and then immediately recombine them 1:1, and you'd get your original L and R signals back. But I'm not sure what benefit that would give you, other than the satisfaction of knowing that you can do it. You don't have nearly as much freedom to process the "M" or "S" signals in that situation (although this approach has been used to reduce noise in fringe FM reception, since the "S" channel in that particular situation is generally the main source of audible noise).
--best regards