a Steel Box Section Build with SBR20 & Ballscrews Plus a few questions

[EddyCurrent]

Take a look at this diagram, you can see that in your design the spindle would hang out from the gantry well past the bearing blocks.

http://www.cncroutersource.com/do-it...NC-router.html

It's usually regarded as X being the rails mounted on the frame, Y being the rails mounted on the gantry, Z where the spindle mounts.
Two motors on X works fine, just remember you would then need to, rate your power supply accordingly, purchase another stepper driver, add 2 home switches. It's generally regarded as advantagous to connect the motor to the ball screws using timing belts, the pulley sizes can also be selected to increase or decrease speed so that your stepper motor is working in it's optimum range while the cutting speeds are also optimum.
You might need this as you indicate 1605 screws all round, I'm using 1610 on X and Y which seems right using 1:1 ratio of pulley