I haven’t tried clamping them, sounds like a plan.
I did removed the X-axis makerslide and swap them over front to back, but no difference.
I made a jig from some spare makerslide to measure the Y-axis rails and they are parallel.
The real test is a diagonal measurement as even with the y-axis rails parallel, one could be further forward than the other if the 20x20 extrusion is not at 90° to the y-axis rails.
I’ll get back to this later after I’ve had my overdue, daily walk.