I haven't used dual z drivers since I really see no need even though I use 2 motors for Z on most of my printers, one driver is usually powerful enough to drive two motors in parallel. But regards Marlin, in addition to uncommenting dual Z you'll also need to define the Z2 driver pins in correct pins.h file (probably pins_SKR13.h or something). I think renaming the E1 pins to Z2 pins might do the trick.