As an update and after much testing and retesting I have refined the udev rules :
KERNEL==“ttyACM0”, SUBSYSTEM==“tty”, SUBSYSTEMS==“usb”, ATTRS{idVendor}==“1ffb”, ATTRS{idProduct}==“008a”, ATTRS{serial}==“00031189”, MODE=“0666”, SYMLINK+=“TTYS_001”
KERNEL==“ttyACM2”, SUBSYSTEM==“tty”, SUBSYSTEMS==“usb”, ATTRS{idVendor}==“1ffb”, ATTRS{idProduct}==“008c”, ATTRS{serial}==“00053419”, MODE=“0666”, SYMLINK+=“TTYS_002”
KERNEL==“ttyACM2”, SUBSYSTEM==“tty”, SUBSYSTEMS==“usb”, ATTRS{idVendor}==“1ffb”, ATTRS{idProduct}==“008a”, ATTRS{serial}==“00031189”, MODE=“0666”, SYMLINK+=“TTYS_001”
KERNEL==“ttyACM0”, SUBSYSTEM==“tty”, SUBSYSTEMS==“usb”, ATTRS{idVendor}==“1ffb”, ATTRS{idProduct}==“008c”, ATTRS{serial}==“00053419”, MODE=“0666”, SYMLINK+=“TTYS_002”
This guarantees that the device 008a will always be TTY_001 and device 008c will be TTY_002 wherever I plug them in. Of course you will have to increase the array if more than two devices are seen.