Help needed configuring for two Z steppers

I downloaded Marlin firmware version 1.1.0 RC 8 and flashed this to my laserBot laser machine which has a MegaPi controller board. The laserBot is now working fine.

I have a 3D printer frame sitting in my shop not being used which is the classic i3 Prusa type with the dual Z axis stepper motors. I am trying to convert this into a laser machine by installing a MegaPi controller board and flashing the same Marln firmware RC 8.

I uncommented the line in config adv.h

Z_DUAL_STEPPER_DRIVER

...but got a compiler error: pins.h:449: error: 'E1_STEP_PIN' was not declared in this scope.

Does anyone know how to resolve this?

My understanding is that if I can enable a second Z stepper, the 4th (and last) remaining port on the MegaPi will work the second Z stepper. I have yet to test that theory however.

Thanks...