The multiple threads on cardioids at 180 degrees got me thinking.

Wouldn't the ultimate mic for this purpose be the multi-pattern microphone
sending output from both the front and back capsules as a stereo pair?

Not that I could think of a valid use, but it would have the advantage of
precise capsule alignment.

The ultimate in this approach is the Soundfield microphone, which can
deliver mono, stereo and/or surround signals, all with effective
directional patterns that can be chosen "after the fact" in
post-processing.

The closest product that I know of today to what you've described is
the Pearl TL 4 or TL 44. There is also a dual version with four
discrete cardioid outputs, the DS 60. http://www.pearl.se/estart.htm

The original remote-variable-pattern condenser microphone was
manufactured by Neumann as the model M 49. It wasn't actually their
invention, though; it was developed and patented by Dr. Grosskopf at
the NWDR, and the rights to manufacture it were assigned to Neumann. It
may have been the best-sounding large-diaphragm microphone they ever
made, when used by someone who knew it well, such as the RCA Victor
classical engineers in the 1950s. See www.neumann.com for
details--choose "Products", then "Hist. Microphones" the click on the
thumbnail photo of the M 49 and choose the last PDF in the list.

Using the same principle, AKG and Neumann both offered stereo
microphones (I believe AKG still does) with remote pattern control, but
they require 12-conductor cables. In a microphone of that type the
halves of each capsule are matched to each other, then the complete
left and right capsules are also matched. Neumann's first microphone of
that type (the SM 2 and SM 23), interestingly, used small-diaphragm
capsules--the same as in the KM 56. Later this was superseded by a
model SM 69 that used the larger capsules of the U 67. Then the tube
amplifier was replaced by solid-state circuitry (SM 69fet).

Neumann discontinued the SM 69fet just a few years ago, but they still
offer the USM 69 which has the same capsule head with fewer patterns,
and the patterns are selected at the microphone now rather than
remotely. (That certainly simplifies the cabling.) In the 1970s there
was even a quad microphone (QM 69) based on the same capsule head, with
four separate balanced cardioid outputs--a person could matrix those
signals into almost any form for stereo, but quad wasn't any more
successful than surround is today, and I don't think they ever sold
more than a few dozen of those.

--best regards