Several projects inspired me to try the CoreXY belts on two planes - one stepper and its belt gets one plane, the other stepper and its belt gets the other plane.
The sole problem I saw in prototyping mine is that the belt comes close to rubbing when the Y axis bar is farthest from the steppers.
I solved that by using two different size pulleys - both made with flanged bearings, but one pair was smaller than the rest, so I was able to locate them such that the motion critical paths are all at 0 or 90 degree angles where they are supposed to be, and the pulley sizes are different enough to prevent rubbing.
I have confirmed that the belts directly moving the Y axis lie on the X axis, which should guarantee linear operation.
The attached pictures show OpenBeam 500mm 20x40 and 20x20 beams forming the mechanism. The bigger bearing size is F625ZZ (5x16x5), the smaller pair are MF105ZZ (5x10x4 mm).
I like the mechanism a lot - I've been doing rapids at 500mm/s with no trouble using Beaglebone+CRAMPS (it may end up being slower after putting my hotend on, of course).
I did mock it up as a full cube, by my first attempt was not rigid enough, and it vibrated the structure a lot.
