If you need an I2C communication between 2 DUEs, one in Master mode and the other one in Slave mode, care must be taken with pull-up resistors. SDA/SCL have already pull-ups of 1K, unlike SDA1/SCL1 which have no pull-ups. Hence you want to connect DUE1 (SDA/SCL) to DUE2(SDA1/SCL1).
There are numerous threads in the DUE sub forum with the I2C topic because there is no glitch filtering provided by the uc either on SDA/SCL or SDA1/SCL1. A 100 K resistor in serie on the SDA/SDA1 line and idem on the SCL/SCL1 line fixes a part of the issue.
There's also this thread among others if you use the wire library:
https://forum.arduino.cc/index.php?topic=308479.0
Plus you can find example sketches using only TWI registers with a very useful I2C bus reset function to be used whenever the bus hangs.