That's EXACTLY what my connector looks like right now, lol. Happened 3rd day into a filament that needed a bit higher temperature to print well.

OP, was your board fine? Haven't had a chance to pull it apart and test yet.

Good catch. I'll report back results.

I'm pretty sure I had it in there before I switched back to 1 Z for testing but I'll add back and test again tonight, just in case.

Here is homez.g. Homeall.g hasn't been updated yet.

[[language]]
;DUAL Z
M584 Z2
M854 U3
G1 S1 Z-320 U-320
G92 Z.04
M584 Z2:3

This me TWO days to figure out why my machine was grinding when homing. Then I realized on EVERY home move the right endstop hit before the left. If I moved the bed down 30MM or more this would happen.

While in this configuration, the motor hooked up to the original Z plug moves faster than the one in the U plug.

Any ideas on why this is happening?

Note: When tested with both motors in Z plug, homes and moves normally. I DID put the jumpers on as for a 1 Z motor hookup.

[[language]]
; Configuration file for Duet WiFi (firmware version 1.17)
; executed by the firmware on start-up
;
; generated by RepRapFirmware Configuration Tool on Sat Mar 18 2017 20:32:40 GMT-0400 (Eastern Daylight Time)

; General preferences
M111 S0 ; Debugging off
G21 ; Work in millimetres
G90 ; Send absolute coordinates...
M83 ; ...but relative extruder moves
M555 P2 ; Set firmware compatibility to look like Marlin

;DUAL Z
M584 X0 Y1 Z2:3 U3 E4

M667 S1 ; Select CoreXY mode
M208 X0 Y0 Z0 U0S1 ; Set axis minima
M208 X300 Y300 Z320 U320 S0 ; Set axis maxima

; Endstops
M574 X1 Y2 Z1 U1 S1 ; Define active high microswitches
;M558 P1 X0 Y0 Z0 H5 F120 T9000 ; Set Z probe type to unmodulated, the axes for which it is used and the probe + travel speeds
;G31 P600 X0 Y0 Z0 ; Set Z probe trigger value, offset and trigger height
;M557 X15:285 Y15:285 S20 ; Define mesh grid

; Drives
M569 P0 S1 ; Drive 0 goes forwards
M569 P1 S0 ; Drive 1 goes forwards
M569 P2 S1 ; Drive 2 goes forwards
M569 P3 S1 ; Drive 3 goes forwards
M569 P4 S1 ; Drive 4 goes forwards

M350 X128 Y128 Z128 E16 I1 ; Configure microstepping with interpolation
M92 X800 Y800 Z3200 U3200 E138.4336 ; Set steps per mm
M566 X600 Y600 Z24 U24 E300 ; Set maximum instantaneous speed changes (mm/min)
M203 X30000 Y30000 Z300 U300 E1500 ; Set maximum speeds (mm/min)
M201 X2000 Y2000 Z100 U100 E4000 ; Set accelerations (mm/s^2)
M906 X800 Y800 Z800 U800 E800 I30 ; Set motor currents (mA) and motor idle factor in per cent
M84 S30 ; Set idle timeout

; Heaters
M143 S260 ; Set maximum heater temperature to 260C
M305 P0 T100000 B4388 R4700 H0 L0 ; Set thermistor + ADC parameters for heater 0
M305 P1 T100000 B4138 C0 R4700 ; Set thermistor + ADC parameters for heater 1

; Tools
M563 P0 D0 H1 ; Define tool 0
G10 P0 X0 Y0 Z0 U0 ; Set tool 0 axis offsets
G10 P0 R0 S0 ; Set initial tool 0 active and standby temperatures to 0C

; Network
M550 PBlack Widow ; Set machine name
M552 P0.0.0.0 S1 ; Enable network and acquire dynamic address via DHCP

; Fans
M106 P0 S0.3 I0 F500 H-1 ; Set fan 0 value, PWM signal inversion and frequency. Thermostatic control is turned off
M106 P1 S1 I0 F500 H1 T45 ; Set fan 1 value, PWM signal inversion and frequency. Thermostatic control is turned on
M106 P2 S1 I0 F500 H1 T45 ; Set fan 2 value, PWM signal inversion and frequency. Thermostatic control is turned on

; Custom settings are not configured

It works!

Disclaimer: I haven't printed with this yet. Still waiting on my bed.

Took some tweaking though.

I may or may not have had to make some of the changes I made. Someone else can confirm.

Added the following to config.g:

;DUAL Z
M584 X0 Y1 Z2:3 U3 E4

Wherever something for Z was defined, I added the same for U. (I'm unsure if this was necessary, as I did this during troubleshooting.)
Example:
This: M350 X128 Y128 Z128 E16 I1 ; Configure microstepping with interpolation
Became: M350 X128 Y128 Z128 U128 E16 I1 ; Configure microstepping with interpolation

Added, as it was missing:
M569 P4 S1 ; Drive 4 goes forwards

Commented out the entirety of homez.g and added:
M584 Z2
G1 S1 Zxxx Uxxx
G92 Zxxx
M584 Z2:3

Pretty sure that was everything!

Awesome! Should have my board on Monday. Once I've got it all working normally I'll give this a shot and report back.

Thank you!

My Duet is in the mail

Any update on this?

Haven't ordered my board yet (still working on other parts of my D-Bot build) but I'm heavily leaning towards it.

Has this ability been implemented yet?