Thus, both master and slave nodes the switch on the Arduino Uno Click shield MUST be placed in PROG position when programming, and on UART position when running on DMX mode. There is no serial ...

with a DMX controller you are no longer in master or slave mode but DMX mode. to do what you want, you simply assign the two lights to the same address. you can assign as many lights as you want to the same address and they’ll all be in sync and follow

Connect a DMX cable from the DMX controller to the DMX IN of the first fixture, and a DMX cable from the DMX OUT of the first light to the DMX IN of the second. Put both fixtures into DMX channel mode (for this example, set it to 4-channel mode), and then