That makes sense. I thought of doing this as well, have a used dual master but need a rebuild kit.
I didn't understand the "plugging one circuit" concept. Still don't. Others may give their opinion as to whether one circuit would do the entire trick.

My plan was to split the dual - fronts and rears and leave it at that. The complete dual diagonal setup requires new 4 pot calipers, and different backing plates for the fronts, as well as the additional plumbling (new lines to to fronts and the rears).
Crille at Veteranproduktor in SC has the mc rebuild kit, but I have yet to determine that the one I have is rebuildable. He can probably get you a MC as well.
If you can't find a serviceable dual MC with the vertical mounting holes, I hear tell that you can get or make an adaptor plate for the later (1800 and 140) MC which has horizontal mounting holes. I see these (horizontal mount mc) on ebay now and again, but rarely the one that's a direct bolt-in to a 122.
Let us know your progress.