MartyMacGyver:
Using I2Cscanner (Arduino Playground - I2cScanner) I can easily get the address of either device (just testing them independently) using the Mega and pins 20/21. The Due fails to see these devices with the same program via the same pins 20/21. No additional pull-ups were used (my understanding is that the appropriate resistors are already present on both boards). Since the peripherals are running at 3.3V Vcc shouldn't be an issue (if anything, I'd expect the Mega to have issues with that, NOT the Due!)
I had the same issue with a digital pot from Microchip. The pot is -according to the datasheet- 3.3V compatible but I2CScanner can't find it, it work fine with an Uno (running the pot at 3.3V).
However, I AM able to write to and control the pot from the Due , so in the end it was not a problem. The moral of this story is that it might still work even if I2Cscanner can't find the device.