I thought I2C could do up to 10 meters, if clock rate was 9600 and had pull up resistor.
The boards are only 1.5 inches apart in the machine, and the master will be very close by as well, so total distance of I2C bus would be 1.4 meters.
What is the advantage of the Arduino Duo? Haven't heard of it before, though it is probably too late in the game to change now.
If I2C won't work, any other communication methods that you guys can recommend? I could do a big parallel cable, or ribbon, as well...