Design Help Pt2 Required for CNC design/Build

[Jonathan]

You may regret one ballscrew some day.

No, you will regret one ballscrew some day. So many times people start off saying they'll use one ballscrew and a couple of weeks later we manage to persuade them to get another!
Just imagine what happens when you apply a cutting force with the spindle near the gantry side (i.e. at limits of Y-axis travel). It's going to be at least 300mm from the support (the ballnut) so you've got a large turning moment which is only resisted by the stiffness of the rails. Combined with having the X-axis bearing blocks close together the deflection will be quite a lot, especially since the fixing of the supported rails to the rails is relatively weak. A 0.2mm deflection (for instance) may not sound like much, but that will affect the finish you obtain with wood and stop you machining metals.

Two ballscrews also lets you use the design I suggested earlier, without gantry sides, which saves a bit on materials I reckon and is definitely much stronger.

I run my machines with one shared driver per two parallel steppers on X axis and never had any problems with this at all.

Ooh that's interesting / unexpected. Are the steppers mechanically linked (timing belt or whatever)? What steppers and drivers are you using?