Processing doesn't like reading Linux USB devices like /dev/ttyACM0 or /dev/ttyACM1.
The solution is easy, just symlink the devices like this...
ln -s /dev/ttyACM[x] /dev/ttyS8[x]
You can put any number after the ttyS, although it is possible that ttyS0 and other low numbers are defined, numbers in the eighties are (almost) guaranteed to work.