Sorry I did not explain. Putting the windings of each motor in series increases the inductance and that limits the maximum speed / torque in the same way as wiring an eight lead motor where parallel connection of the windings will give more high end torque particularly with PWM drivers. Putting the motors in series solves the current sharing but at the expense of torque at the higher stepping rates. That is why I suggested trying to keep lead lengths and resistances the same for each drive if you connect the motors in parallel.

My other concern is with the way each step is translated into physical movement and the way that movement has an affect on the other side of your gantry. If your machine is a light build then the two sides of the gantry are coupled by a structure that can flex a little bit and this has to absorb the differences due to motor non linearities and those of the screw - we are talking differences between the two sides. This might induce resonances if you are unlucky. However since there is no cutting loads to consider you should be OK but if the inspection head is a camera then it may need a short period between steps to allow the structure to settle. This used to be a problem with the old optical flicker comparators for PCB work.

OK I missed the reason for wanting to drive the other side was to keep the gantry legs in step. A simple way would be to use a timing belt with a pulley on each lead-screw. This would keep the screws in step - this is a common configuration to solve the crabbing of the gantry. The motor or motors could then be added to the timing belt path if this simplifies the layout of the modification.

Hope this clears up any ambiguity in my response to your question.

Regards - Pat