I have both screens working at the same time now, and I got the Sharp to be inverted (black-on-white) which I think is easier to see on that reflective display. Right now, I'm basically just using the Waveshare python epaper library directly to test the screen out, but I plan to write up a nice little CLI utility (or modify one that exists) in order to display any arbitrary thing on it, including piping std out. That might be the first thing I actually write on the device itself once it's all done.

I tried using the same SPI interface for both, but I couldn't get it working, so I enabled the second SPI interface on the Pi and am using that for the epaper.

This is the wiring I came up with use both these screens at once: