Using two 3216 displays together is an almost trivial hack. I wanted to make sure it works fine with the Wise Clock 4 board. The video below shows the new board running both "Night and Day" and "Wise Clock 3" (merged into one sketch, of course).
In practice, one can now build the "Dual Screen Wise Clock 4" with the two displays placed on opposite sides (back to back), or one next to the other (as shown in the video). A crazy idea would be to have 4 displays placed in a square :)