I think there might be another way of doing it with a Model M, but would likely still require two boards for parts.
Idea is that it might be possible to cut the plates to the outside of the chosen separation line (so the hammer "wells" and guides remain intact for the edge keys - one side from each board) while extending the cuts for the membranes a half inch or so further out from the plate cuts.
Then make mechanical, clamped contacts between wires and the cut traces on the membranes (possibly requiring some conductive glue to make extra sure the contacts are electrically sound and won't corrode with time) then string these wires to a socket arrangement, one for each side, so that you can then use a cable to connect the two halves (actually, three parts, as I was going to put the Number Pad and possibly Cursor Key section on separate module as well). This is if going for a separated board, or alternatively one could just connect the repositioned sections internally if going for a single case design like the Marquardt or the Northgate ergos.
A lot of work, but not as kludgy, I think, as folding the membranes, using two controllers, and two USB connections, etc. My way would be more complicated, and yet more elegant at the same time, IMHO.
If you wanted to do even more work, you could re-arrange some of the controls, w/ two boards to part from - including using more of the active key wells, ala Ripster's mod, and even cutting up both cursor inverse-Ts so that you can move them to the outside bottom of both main modules as with the M15, moving the rest of page navigation section to the Numbpad module or elsewhere on the main modules.
A lot of possibilities present themselves if the mechanically connected jumper wire idea proves out.
Anyway, I was thinking along these lines as an experimental mod, but not sure I'll ever get around to it.