If you are using a traditional gantry design (sides of the gantry extend below the cutting table and are tied together underneath the cutting table), then it makes more sense to use a single, centered drive screw.
ACME screws have an energy transmission efficiency of only between 35% and 48% -- so the extra screw is going to eat up a lot of your motor's torque.
The extra cost (don't forget belts, pulleys, end bearings, etc.) make it even more unattractive.
If your gantry is some other design (e.g. an overhead gantry riding on elevated rails), then that might force you to go to the two-screw method.
-Chris |