Thanks Jonathan.

I can see that you couldn't home each motor separately with a parallel connection but I'm a little surprised that you would need or want to (at least after the initial machine set-up). Surely the two motors should be taking identical steps and therefore remain square on it's own?

Obviously no two steppers will be perfectly identical but the difference in step size between two motors must be tiny for the dual motor set up to work at all. Even if the two steppers did have a non-trivial difference in step size surely the worst case would be wherever is furthest from the home position getting worse in a linear fashion the further from the home position.