I undid things one by one and found that when I added the Serial.begin, it stopped lighting up. That seems like the complete opposite of what you would want, since that would be for debugging...
But whichever matrix I define first is the one that lights up. They both report 0 to the serial interface in numPixels (when serial is enabled), but perhaps that's because they don't actually light up if I have the serial turned on?
Is there a way to print to the console below the code window instead of specifically turning on the serial console? I mean the one where the errors display, just below the code. My Google Fu has failed me in this search so far, but I'm still looking...