Here is a CNC router design with high cantilevered rails and feed tables
for full sheets of plywood. The P/C router is tucked in under the y axis to reduce the moment. CNC router parts are used for bearing trucks.
The y axis has got a 4" CRS rail I am planning to use (2) 2" flat bars so that I can adjust them for linearity. I feel drilling two holes between the rails will provide a spot to use a spanner to spread the rail for adjustment.
I would recommend against using 2 2" bars instead of a single 4" bar -- the 4" bar will provide significantly more stiffness to your machine, and there is already adjustment built into the carriages, so adjusting the rail is fairly easy with a quick loosening of bolts and a tap with a hammer. Otherwise, this is an interesting design.