The answer was in: Extruder motor/ Z axis issue
This board has 9 drivers for 8 axes where driver_2 is mapped as motor2_1 and motor2_2. I did not look at the PCB silkscreen, I only used the driver table in the manual which only has 8 lines not 9. So there is no need to configure stepper_z1 on this board, it seems to be designed as dual in hw already.