Thanks to my TwoBitChunker program, I've got my font data. Put it in a pair of files, started to write the routines to use it into the main clock source.
The right screen now shows the seconds digit in my large font. The font don't look quite right (I'll need to adjust it's conversion), but it works.

It too me a LONG time to figure out that the Arduino GFX library's drawBitmap expects PROGMEM data, not RAM data. It would have been really nice if that was documented. I understand why it's done (it avoids needing a RAM buffer), but what a mess that was to find.
It's a clock! It now displays the time (in 12h format) in large numbers across the two LCD screens.
Fixing a copy & paste error that caused the hours display to be wrong. Changing the calls from rightDisplay to leftDisplay for the hours function, even though it didn't cause functional problems.