Core Sound once planned to market a preamp which mux'd 4 channels to 2, with software demuxing of the resulting files back to 4, but it never panned out. Let me see if I can find a link.
That required a 96 or 192kHz SPDIF input recorder, with the Microtracker the small recorder option targeted for doing so at the time. That was prior to the availability of lower cost analog input multichannel recorders which squashed the market for it, the R-44 came out right around that time, then cheaper Tascams and Zooms. Plus it was about as large as the 4 and 6 channel all-in one recorders now available, and probably around the same cost or more.
But I've wondered if the same could be done robustly enough with a mux'd analog output to withstand the D>A>D conversion necessary to use a small recorder which only has analog inputs. Not sure if the real world bandwidth of the available small recorders would be up to it though. The now discontinued Tascam Dr2d can record 4 channels at 24/96, dunno it's realworld S/N capability or frequency range though. But it could potentially capture 8 channels at 44.1 that way, and the ubiquitous Sony M10 or the like 4.