Having changed to a controller with 6 steppers, I have the option to run the pair of z-motors in three ways:
1) In series (per the original kti)
2) In parallel
3) One in slave mode on a separate stepper
I used Nophead's kit from a few years ago, where the manual described wiring the motors in serial.
So far as I know that's because being in serial uses less current (but requires a higher voltage), and that the only advantage of in parallel is higher torque, which is not beneficial on the z-axis.
Just wondered how other people are wiring up theirs on the Mendel90?