The biggest problem I see with that design is the Z axis. You've made it rise up way past the gantry, but this doesn't get you any usable travel, unless you're planning on machining foam with extremely long tools. If it was half as long, it would work just as well with normal tools, and be more stable. Raising the rails up a little on all the axes would allow you to position the ball screws between them, which would work better. Using dual motors on the X axis is something people do to avoid the "stuck drawer" effect which happens when you push a sliding stage off-axis, as you seem to be doing throughout this design.