My mistake those are 20mm Hiwin rails in my drawing.
Do I use 2 100x200 in a one on top of other configuration or 1 across from each other, either way we are making a bigger footprint in the overall design and the X axis will have to be longer for the same inside working area.
I do like the 1 across from each other with 4 20 mm hiwin rails as it will reduce the Height but will it be better supporting a Z axis hanging 16" underneath plus 6" inches of travel?
My design has 14" of support above the Z axis. The 4 20 mm Hiwin rails will ad considerable strength to the 40 x 80 extrusions.
I will mock up a couple pics.