Thanks for the comments. My existing machine is pretty poor anyway, but only has a single X motor so I have no personal experience of the positioning issues and it's interesting to hear what problems other people see. Nearest I get to it is my 3D printer which uses two motors to drive the Z leadscrews; those motors are just wired in parallel from a single driver and within the tolerances of a 3D printer, I have never had any problems with drift of home position between them. However, it's all a bit different with two drivers/two motors, of course, as the drivers themselves can theoretically get out of sync with each other at the microstep level.

Anyway, I quite like the idea of grafting in an Arduino-based solution - I've done a bit of work with them both as the 3D printer controller and programming the microcontroller itself to use on a bare board. I'm sure that that could make quite a neat solution.