So a 2.5D router?
I have an OX, which is belt-driven and fails harmlessly (except to the workpiece) if one stepper fails to move in sync with the other on the Y axis which is the only axis driven with two steppers. I chose the option of one hardware port routed to two external drivers. I have the STEP, DIR, ENABLE, and GROUND lines from one port on my controller board (not using a stepstick or direct motor control ports) connected to two external stepper driver units.
However, usually, you want to go out of your way to use a single motor on an axis if possible, and that’s even more true with ballscrews. If you had a failure that caused one of two steppers to fail (and there are lots of opportunities for that) is there any chance that the racking would cause damage to one or both screws?
I would use a single stronger stepper and a timing belt rather than multiple steppers. With the mechanical advantage of the ballscrew, timing belt would behave well WRT cutting forces. You could use a pair of 60T or 80T timing pulleys between the ballscrews and the forces on the belt would be essentially insubstantial, and you would know that the screws would run in sync. You could use two pulleys, one on each end of the screws, and have redundancy in case of a belt failure.
I can’t tell what you are making the frame out of, but it looks thin relative to your screws and rod. I’d expect compliance in your frame to be more of an issue than any timing belt stretch if you did that.
Have you considered bolting rail for open pillow block bearings to a frame element for your dual-ballscrew axis?
I know these ideas aren’t what you came here to ask, but I couldn’t help myself.