After installing a fresh default image on my second Pi, I noticed it had a config file already there too, but with a note at the top saying not to edit it and that the config is in the firmware folder.
Anyway, I was able to get both pi's running and connected through Ethernet with the web interface using your installation on the main pi and the default image on the second pi using the web interface.
I hope I'll be able to work out enough about the web UI that I can make just the spectrogram appear on the second screen, but I have no idea if it's feasible to even have two web uis running at once.
Scott W1LD