Followers to these threads know I've long been interested in exploring ways to decorrelate or otherwise spread a single recorded rear-facing OMT ambient channel across two or more mixing or output channels. The intent being to have that ambient depth information portrayed in a more enveloping and spatially open way in both the 2-channel stereo mix and for multichannel playback where that signal is reproduced by multiple surround speakers arrayed around the back and sides of the listening space. An equally important additional benefit in the 2-channel stereo mix is to keep that monophonic rear-facing ambient signal from interfering with the forward-facing center microphone or coincident pair tasked with picking up the primary direct sound of interest and reproducing that with tight-imaging in the center of the playback stage.
In other words, the goal is to spread out a monophonic rear-ambient signal such that it wraps around the back of the listener perceptually, and doesn't dominate the center of the stereo mix where the forward-facing direct sound should be clear and dominant.
We've discussed here previously things like mult'ing the mono-rear-facing channel to two channels, inverting polarity on one channel and panning them left/right - which is the same as routing the monophonic rear and front-facing channels through a Mid/Side matrix with a front-facing channel assigned as Mid and the rear-facing channel as Side. That works decently well sometimes, but not always, and causes some low bass cancellation on the right side. Better should be a 90-degrees quadrature phase shift in opposite directions to both channels rather than flipping polarity on one side only (a la matrix-surround encoding), which kuba e has tried and reported as being an advantageous approach. And we've discussed a few other options as well, but most "stereoization" techniques and plugins introduce various artifacts, reverb, or other unwanted stuff.
With all that in mind, I'm intrigued to find the Schoeps
Mono Upmix plugin available, as it looks like it might be a good solution for addressing this.
Other than using it in stereo output mode, which may be the most appropriate way to use it, It's LCR output option apparently routes the monophonic input to Center and only the upmix spatialization (extracted ambience and generated early reflections of set to produce them) to L/R. For a stereo mix it might prove advantageous to use it in LCR mode, discarding the Center output and using just the L & R outputs.
It might also be useful to "slightly stereoize" a single forward-facing center channel to help spread it slightly and blend better into a 2-ch stereo mix, similar to use of a coincident stereo pair in place of a single forward facing center microphone, although without providing true left/right imaging like a coincident center pair provides.
https://www.plugin-alliance.com/en/products/schoeps_mono_upmix.html