Hi, correct me if I'm wrong, but when you have two different size balls in a ball nut the smaller balls do not carry any load but act as seperators for the load carrying balls and are placed alternately in the line so that they can counter rotate between the load carrying balls without being hard driven against one another as happens normally when balls of the same size butt against one another.

This is only a wear reduction solution and you still need two nuts to have zero backlash.
Ian.