This is an interesting thread. Some would think that 2 bodies and two lenses would do fine, however it depends on how well you can "see" photographically while using one focal length and seeing another chance with yet a different one. It also depends upon whether or not you're on a "photo mission" or merely doing errands in your home town. Those that aren't trained to be able to "see" several focal lengths at once often shy away from carrying additional lenses. I on the other hand am a bit of an "oddball" and see several compositions from super-wide to telephoto with the same subject matter. It's something that was learned by starting-out with only prime lenses (no zooms), which I fear is something that has long been lost in this, the era of auto-everything P&S.
Has anyone here any experience with such things as the Leica lens carrier that goes underneath the base plate and screws into the tripod socket? I use mine once in a while when not using the Motor-M's I own. A 90 Apo makes a nice hand grip on occasion while holding the M6 over my head with a 21 or 28 attached.