Well...now I have two Arduino Leonardos - one for a manual and one for the stop tabs. MIDI Monitor shows that both are happily generating messages, on different channels. MIDI Monitor can see both Arduinos (both are listed in the source selection), but GO only seems to see a single Leonardo, and is not accepting messages from the new one.
If I disconnect either one, the other registers with GO properly, so I don't think it's an issue with the Arduino programming or hardware. I had hoped GO would "blend" them into one source, using the different channels to distinguish between the message and thus allow both.
Any thoughts about how to go about this?