Serial From Pi using Py to Arduino Due then to PC and SSC-32

  // By default, the last intialized port is listening.
  // when you want to listen on a port, explicitly select it:
  SerialUSB.read();

Just how many SerialUSB instances are there? How can there be another one listening? You do no make the SerialUSB instance the listening instance by throwing away a byte from the buffer.