”Double M/S” is an improved version of the well-known M/S stereo recording technique. In addition to a front-facing cardioid or supercardioid ”mid” microphone and a figure-8 ”side” microphone, a rear-facing directional microphone is set up. The front- and rear-facing microphones share the signal of the figure-8 microphone so as to form two complete, back-to-back M/S systems.
One M/S system then provides the three front channels (the center channel signal being provided directly by the mid microphone of the front system), while the other system provides the two surround channels. An arrangement of this kind allows flexible processing of the stereo surround image width and post-production adjustment for both of the M/S pairs.
It's even easier with the free SCHOEPS Double MS plugin.