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I have built many MendelMax 1.5's, and I always wire them in series. But, in my experience, if there is binding on one of the motors, they can still get out of sync. I fix the binding issue, so they usually don't get out of sync. Still, its easy for some idiot to grab one of my z-axis motors and turn one of them while the printer is off, un-syncing them..
I have not tried parallel wiring the z-motors.