I do not think the Arduino Due is supported by the Wire Library. Ideally I need to setup both laptops with one connected to the Arduino Due and the other connected to the Arduino Zero and debug it all. Swapped out the Arduino Due with an Arduino Mega2560, Leonardo and UNO.

I tried it with both sets of pins & i did have pull-up resistors on the 20 & 21 pins.

From does seem like it would be useful to add that information to the Wire library reference page. Library examples page implies the Wire library is not compatible with Due I've reported it to the Arduino developers: It does seem like it would be useful to add that information to the Wire library reference page. Adding two pullup resistor on SDA1 and SCL1 lines is required for using Wire1. While SDA and SCL have internal pullup resistors, SDA1 and SCL1 have not. SDA1 and SCL1 can be controlled using the Wire1 class provided by the Wire library. Support TWI communication using the Wire library. There is the question of which set of pins would be supported. My hunch is that with the Arduino Due having Two sets of TWI pins Its example sketches are functionally identical (other than a couple of device IDs being different). The hardware package of the Due, Arduino SAM Boards does contain a Wire library. I don't have any experience with the Due but it definitely seems wrong or at the least unclear what that sentence means.

Arduino due wire library update