Arduino IDE monitor multiple USB ports simultaneously
If you are only interested in monitoring the output, a simply python script can be run to monitor the port. This sample takes an input of the port number(assuming they get named ttyUSB1, ttyUSB2, ect...  Or you can modify it to be a hard coded port.  Also make sure the baud rate matches with what your arduino is running (115200 in the example)

You can then run this script in two separate terminals, one for each port

#!/usr/bin/env python3


import serial 
import sys

if len(sys.argv)&lt;2:
    print("Please give serial port number")
    exit()
port=sys.argv[1]
ser= serial.Serial('/dev/ttyUSB'+port,115200,timeout=5)

while True:
    try:
        print(str(ser.readline().strip()))
    except:
        print(sys.exc_info())
        ser.close()
        exit()


Start the program like so 

python3 ./programName.py 1 (or ./programName.py 0, etc)

The number dictates which of the ttyUSBx devices you are trying to watch (will match with what you see in the arduino IDE).