Two Mid/Side pairs as near-spaced stereo array providing variable pattern/microphone-angle-
This is a very interesting setup possibility for a near-spaced stereo pair. The more I think about it, the more I like it. Near-spacing distance between microphones remains constant, yet this allows one to vary both pickup-pattern and microphone angle. Choice of pattern and angle is linked, just as with standard Mid/Side. First thought and most straightforward would be to set this up with both Mid/Side pairs parallel to each other and pointing directly forward. However, pointing the two Mid/Side pairs such that they face directly to either side instead of forward offers a far more useful range of pattern/angle combinations. In which case, adjustment of the Mid/Side ratios allows one to vary pattern/angle between forward-facing parallel figure-8's (Sometimes described as a Faulkner phased array) through cardioids with a 180 degree angle between them. At Mid/Side ratios in between those extremes one will derive supercardioids with an angle somewhat wider than 90 degrees and hypercardioids at a somewhat narrower angle. Pattern and angle can be varied seamlessly across this spectrum after the recording has been made, with the decision of what sounds best made by ear.
Setup for two near-spaced M/S pairs-
Use two Mid/Side pairs with cardioid Mids. Space the two pairs 17cm apart (or 20, 25, 30cm or whatever near-spacing you want), and angle them such that the Cardioid Mids are facing directly to the sides (180 degrees away from each other). Both Side microphones should have their positive polarity lobe facing forward, so flip the Side figure-8 of the Left pair so that it's positive polarity lobe is facing forward instead of rearward.
Record both raw M/S stereo pairs (4 channels total). Let's call the Left-side M/S pair- pair 1, and the Right M/S pair- pair 2.
After recording, you run both raw Mid/Side pairs through Mid/SIde to Left/Right decoders. The Left output of Pair 1 is routed to Stereo Left. The Left output of Pair 2 is routed to Stereo Right. Set the ratio to determine the pattern and angle (both remaining linked, as it always does with standard Mid/Side) from forward-facing figure-8 through sideways facing cardioid. Typically one would use the same ratio for both Mid/Side pairs, but one could use different ratios on each side to choose different patterns and angles for Left versus Right.
Cool multichannel surround / ambience channels option-
The Right channel outputs from both Mid/Side decoders provide rear-facing virtual microphones with a pattern and angle that mirrors the forward-facing microphones on either side. If you understand how to dial in a different Mid/Side ratio for Left versus Right in a typical stereo Mid/Side setup, you'll see how you are able to choose a different combination of pattern and angle for the rear-facing ambience channels.
A few thoughts-
While going through this, after realizing that it makes more sense to point the Mid/Side pairs sideways instead of forward and makign the drawings, I realized that there is nothing new under the sun. Kudos to Jerry Bruck and Schoepes for basically pioneering this approach already with their KFM 360. This is basically the KFM 360 arrangement except substituting cardioids for the flush-mounted sphere baffled omnis.
Over a decade back, I schemed about of ways of emulating the KFM360 as a lower-cost, headworn HTRF setup. I ended up going with an alternate 4-channel setup which did not use Mid/Side pairs. Still these ideas have been been in the back of my mind since then, and this seems a setup that seems far more practical actual real taper use.