Will likely help to lowpass or EQ cut most of the higher frequencies from the rear-facing capsule to avoid picking up additional noise in that non-correlated frequency range which will not cancel out.
This can be done in post! I call for a new age of experimentation! #IAmNotBored
¯\_(ツ)_/¯
That's definitely a special situation thing.. cool if you can get it to work. Might try it if there is a generator or some other similar noise source directly behind you at an outdoor festival, or maybe a bad back wall reflection inside.
I often EQ the rear-facing channel(s), but since in my case it/they are intended to provide extra dimension and space (rather than attempting rejection) I don't want to cut all that mid/high frequency stuff. I just want it to sound natural and complement the main sound arriving from the front without conflict, with EQ and level set to taste. When I use the same rear facing channel(s) for direct surround playback rather than a 2ch stereo mix, it can help to EQ so that I can raise the surround levels more without the front image being pulled around into the rear. That takes the "EQ to make it natural sounding" thing a step further by cutting presence range somewhat more, typically ending up with something like a loudness curve.