So let's get this sorted out. First plug in only one Arduino and type

ls /dev/ttyACM*

You should get /dev/ttyACM0. Use it in the following command.

Then run this command

udevadm info -a -n /dev/ttyACM0 | less | grep KERNELS

You will get something like:

    KERNELS=="1-1.2:1.0"
    KERNELS=="1-1.2"
    KERNELS=="1-1"
    KERNELS=="usb1"
    KERNELS=="20980000.usb"
    KERNELS=="soc"
    KERNELS=="platform"

Write down the identifier from the first line ("1-1.2:1.0"). This identifies the USB port in your system.

Repeat the above with the Arduino connected to another USB port(s) and find the USB port identifiers. I have "1-1.2:1.0" and "1-1.3:1.0".

Now create a file named /etc/udev/rules.d/99-arduino.rules, e.g.

sudo nano /etc/udev/rules.d/99-arduino.rules

Put the following in:

SUBSYSTEM=="tty", KERNEL=="ttyACM*", KERNELS=="1-1.2:1.0", SYMLINK+="arduino_one"
SUBSYSTEM=="tty", KERNEL=="ttyACM*", KERNELS=="1-1.3:1.0", SYMLINK+="arduino_two"

Add more lines if you plan to use more than two Arduinos. Save and exit.

This means that whenever a ttyACM* device is plugged into the USB port "1-1.2:1.0", a symbolic device /dev/arduino_one is created. And that whenever a ttyACM* device is plugged into the USB port "1-1.3:1.0", a symbolic device /dev/arduino_two is created.

Now disconnect your Arduino and run

sudo /etc/init.d/udev reload

Afterwards connect your Arduinos to the appropriate USB ports and you should be able to use /dev/arduino_one and /dev/arduino_two in the REXduino function blocks. Unless you mismatch the USB ports, you'll always know which Arduino you are referring to.