digits, if a ballscrew can be installed down the center of the moving object, a single larger screw is better than two smaller screws. Ignoring complexity, the larger screw has a higher critical speed.
I'm not familiar with the Mori-Seiki machines that use twin screws, but I'm sure there's a logical reason for it. A properly engineered carriage will not bind even if the force is applied on one side, unless it was designed that way.
A good example of a machine designed to absolutely require twin screws (or twin R&P) is the typical CNC router with a "3-piece" gantry, as I stated earlier.
http://www.thermwood.com/twood_site/...53/M53_516.jpg