This is absolutely beautiful. I'm designing my own CoreXY and came across yours when researching stacked belts vs crossed belts to see if the torque form stacked belts really matters. Does your independent system work reliably? My design is very similar to yours using 4 linear guide rails for the z axis, and 2 for the y, but I hadn't considered doing independent heads for dual extrusion. The design of this machine is phenomenal. The only change I think I would make for mine would be to use E3D's Aero heads for each extruder/hotend.