I don't know if this is just an academic quibble, but is there an issue with the concept of two steppers driving a single, very rigid, axis with no possibility of adjusting their relative position? I'm thinking that the typical micro-stepping driver (although possibly not the latest and greatest) will reset to the nearest full step at switch-on, and there is no reason to suppose that the "full-step" positions on both steppers will align. In the case of the typical both-ends-of-a-gantry setup, there is enough give to allow for this (and we are taking about quite small mis-alignments - a full step with a 5mm pitch screw is only 0.025mm) but auto-squaring takes this out anyway. But if this dual-drive Z axis is going to be much stiffer, is this an issue? Maybe this will be addressed at initial setup?

I believe that some newer stepper drivers will maintain micro-step position across switch off/on, which might help.