Regarding the Due's second USB port, it might work, in particular since it can act as a USB Host as well, but the only way to know is to dive in and try it. Even if you can do it USB to USB, doing it as I suggested (Software UART to Hardware UART) might be more straightforward from a software perspective, and not much more difficult from a wiring perspective. I was only suggesting you use the SoftwareSerial library on the Angular Clock board. On the Due you can wire to any of the 3 "extra" hardware UARTs and use Serial1, Serial2, or Serial3 in software just like you use Serial (so no need for SoftwareSerial on the Due side).
That's a good point re: the ICSP header, you can use those pins just as well as the METERS pins. Here's a mapping of those pins to Arduino signals.
I would use DIG12 and DIG13 for SoftwareSerial, as DIG11 is also connected to the red LED on the Angular Clock. Just replace SoftwareSerial mySerial(5, 6); with SoftwareSerial mySerial(12, 13); in my previous post.
In summary, my recommendation (admittedly biased by my know-how) would be:
- Use SoftwareSerial on the AngularClock board on DIG12 and DIG13
- Use Serial1, Serial2, or Serial3 on the Due
- Wire the power from the Due (5V and GND) to the Angular Clock ICSP header
- Wire the SerialN RX, TX pins of the Due to DIG12 and DIG13 on the ICSP header
- Do the software thing
Side note, you could also wire the Angular Clock Reset on the ICSP header to a Due GPIO pin to enable the Due software to reset the Angular Clock board on demand (by issuing a GND pulse for a few milliseconds), if that's useful to you.