SerialEvent handling when using multiple serial ports
The built in serialEvent() function on arduino only responds to serial port 0, called just "Serial" in the code. To respond to messages on "Serial1", use serialEvent1() on the Arduino Due there also exists serialEvent2() and so on. 

EDIT: the serialEvent() handlers are not on interrupts; they run sequentially in-between loops. If the loop is blocking they do not get run, and they do not interrupt eachother.