Hello

[JAZZCNC]

Just so I'm clear do you advocate one stepper driving the two x axis screws via a belt like the image attached

or two separate motors and drivers? if one is used presumably the motor needs to be more powerful?

Personaly yes I prefer one motor using just the system you have drawn.
My machine is done this way and it completly eliminates some potential issue's that come with slaving motors together. I have used both methods and by far prefer this method.
It's not pritty and slightly more work intially but the peace of mind your not running out of sync and that if one motor stall and the other keeps going the machine not going to try and destroy it's self is well worth the trouble.
The cost is not much less due to belts/pulleys and has you say bigger motor and drive but again there's plus's in that it's less electrical components to intigrate and go wrong plus less Inputs/outputs required so more spare for other things like Probes, etc.

The other BIG differance comes when you you tune the motors.? When slaving motors you effectively have to leave a large safety margin regards motor tuning and velocity/accelleration than when using a single motor.
By the very nature of how steppers work and the fact they lose torque has RPM's rise then the chance of a stall increase's massively and on dualscrew setup stalling at high feeds can be devastating to the machine and your heart plus wallet.!! . . . . With single motor you can push the edge without fear of killing the machine or your self.