The ball-nut doesn't have to be double to be zero-backlash. The best way to find out about one particular type of ball-screw, is to look up the speck-sheet. It usually says how the pre-load is achieved. Usually it seems to be achieved by oversized balls, or by grinding the ball-nut a special way (maybe you could say that there is two nuts in one nut?! (hehe). Thus no double nut is needed. A C2 ball-screw does not have to have less backlash than a c7, but yes I guess it's true, since they probably don't sell a c2 ball-screw without pre-load. I have not seen any though.
I would bet on NSK's, but then again I have never used Hiwin. I don't think I will even buy a new c2 screw though.:eek: