A dual Z axis upgrade with a single stepper is in my opinion a must have for the Ender 3 because this increases the accuracy and reliability of the machine. I have designed one which uses standard parts which don’t need to be modified and it is combined with the frame braces I have designed earlier.
I have added corner pieces without the frame braces if you are just interested in the dual Z axis. I have designed the bracket to be as stiff as possible and therefore it has this odd shape which I am quite happy with.
I have bought all parts from RepRapWorld and it happens to be that the 696 mm closed loop timing belt together with the two 60 teeth pulleys have a center distance similar to the two vertical beams. All holes are slotted so it can be adjusted a bit.
The power supply must be placed somewhere else and I have seen that there are some mods which enable you to do this.
I have made a video about this upgrade in which I show the process.