I haven't actually done this, but I've been considering how to make a dual display variant, perhaps as a display piece for an astronomical museum - one display could show LAST, the other standard time.

To do this, you're going to probably have to come up with some new mounting / case scheme, obviously.

But electrically it's fairly straightforward. Assemble the first clock as usual, with a pi zero w. This display will be display 0.

Next, build an identical board, but leave the 40 pin GPIO header off. You'll be using 5 jumper wires to connect from this board to the GPIO header on the other board (actually, on the Pi that's connected to it). This will be display 1.

The connections required:

Thusly connected, the second board will take power from the first, and share the SPI bus. The two clocks will keep out of each other's way by using different nodes in /dev. You simply need to run two different clock daemons, with one of them using /dev/spidev0.0 (for the display connected directly to the pi), and the other using /dev/spidev0.1 (for the auxiliary display).

You do need to make sure your +5 volt power supply is beefy enough. You should count on at least supplying 500 mA.