I read the guide, but I'm not sure if that is also the way to direct the Duet to use the E1 controller for the second Z-axis not just for the leveling feature.
I just got back to the machine after about a year of letting it sit due to time constraints (new baby and college). Now I want to get it running again with the change.
Yes that is also the way to drive multiple Z motors from separate drivers. Look up M584 in the gcodes wiki page.