I'm not even getting a clock signal on SCL1, just running the example file "master reader"! SCL1 just sits low at 0V (referenced to a gnd pin) and SCL(21) does the opposite (3.3V). I was surprised by how easy it was to pull SCL1 high even with a 100k resistor, as though it is not trying to pull down the line at all, ever. Is my Due faulty? Is there some special way that SCL should be terminated that I missed which is causing this behaviour? I would really appreciate a developer looking into this as my current project was designed around lots of Dues talking I2C.... oh dear 
The previous posts suggest that both the Wire library is in error AND that the hardware has been badly designed with too small pullup resistors and that Dues therefore cannot be connected on a single I2C bus. Please confirm. This is such a fundamental (and disappointing) issue!