I'm must be missing some understanding because I've never seen anyone set up a machine in the way I'm about to suggest but I can't immediately see anything wrong with it...

Imagine a machine with dual x-axis steppers, why can't you just wire both stepper driver control lines in parallel to the x-axis connector on the BOB?

I can see that the signal strength (current) may be a problem as it will be halved but that doesn't feel like it would be insurmountable. If the machine is set up correctly wiring in parallel looks like the same end result as, for example, the slave feature of the PMDX-126.

Am I right in thinking that the step line indicates that the stepper should take a step by going high and then the direction line is high or low depending on the direction of rotation? Is there any other signalling taking place along these lines?