You are using motors with very high inductance and low current ratings. These are a poor choice for a 3D printer because they need high voltages to drive them at speed. With 12V power you stand almost no chance of getting them to work well enough for a 3D printer. With 24V power it would be better, but the speeds achievable will still be low. Use the EMF Calculator at http://reprapfirmware.org/ to work out the speeds at which torque will start to drop.
The two Z motor outputs on the Duet are connected in series because that is generally better for low inductance motors. For your high inductance Z motors, connecting them in parallel would work better. Or, if you are not using the E1 motor output, you could connect one to Z (with the 2 jumpers in the spare Z connector) and one to E1, then use the M584 command to assign them both to Z.