Hi & welcome,
Viewing and focusing of rangefinder cameras with both eyes open only works with 1:1 viewfinders. I am not sure which magnification viewfinders you have in your M6's, but even the highest 0.85x version this will not really work. Both eyes need to see the same size image. With 1:1 finders and both eyes open you should see the frame lines "suspended in space", so as jlw says "you will be aware of what's going on outside the viewfinder".
I like you wear glasses so unfortunately with the R-D1 at least it means I can't use the 28mm frame in this manner as you can't see the full frame lines without scanning. I can just about get away with the 35mm frame but the 50mm is where it really scores. Also unfortunately for most eyeglass wearers using a diaopter correction on the camera instead of glasses will not work very well with 1:1 either, as then both eyes are seeing a different level of sharpness.
The rangefinders focusing is of course is only really taking place using the eye that is looking through the cameras finder and it takes a little time to get the hang of this with both eyes open. Once you get the knack the R-D1 seems repeatable in focussing accuaracy as the image seems to further "clarify" rather than just coincide. See page 4 of the Fast Lenses on R-D1 thread for more on this.
I think that 1:1 viewing is a great advantage, but as I tend on the whole to favour slightly wide angle lenses, especially 35mm on full frame, I think I could sacrifice it for say a 0.72x finder in an R-D2 so as to extend the frames out to 18mm (28 equivalent) and 24mm (37mm equivalent) which would better suit my shooting preferences.
