I don't know much useful about Fermentrack but if it follows the paradigms used on BrewPi you should be able to dictate which Arduino is assigned which logical device using the /etc/udev/rules.d/99-arduino.rules file.
For a multi-chamber BrewPi setup, the process goes like this:
Remove all but one UNO from the USB ports and hubs, then run these commands:
$ udevadm info -a -n /dev/ttyACM0 | less > info.log
$ nano info.log [LOOK for the ATTRS{serial} for Arduino device]
looking at parent device '/devices/platform/bcm2708_usb/usb1/1-1/1-1.3/1-1.3.4':
KERNELS=="1-1.3.4"
ATTRS{manufacturer}=="Arduino (
www.arduino.cc)"
ATTRS{serial}=="85336303532351F0A031"
Repeat for additional UNOS using the same physical USB port (remove the previous UNO, connect the next, etc).
Record all of the serial numbers and note which belongs to which UNO. Then create the rules files, assigning each UNO to a unique SYMLINK name:
$ sudo nano /etc/udev/rules.d/99-arduino.rules
SUBSYSTEM=="tty", KERNEL=="ttyACM*" , ATTRS{serial}=="85336303532351F0A031", SYMLINK+="brewpi1", GROUP="brewpi"
SUBSYSTEM=="tty", KERNEL=="ttyACM*" , ATTRS{serial}=="85334333931351F0F020", SYMLINK+="brewpi2", GROUP="brewpi"
Once the udev rules file is created, disconnect your Arduino and then reload udev before connecting all of the Ardiunos to their respective ports. Note it won't matter which physical USB port you plug the device into, you won't be using physical device names, you'll be using the symlink names.
$ sudo /etc/init.d/udev reload
Finally, go to each BrewPi (and Fermentrack, hopefully) instance and edit its config.cfg file to set the symlink port name as the primary to use:
Eg:
$ sudo nano /home/brewpi/brewpi1/settings/config.cfg
scriptPath = /home/brewpi/brewpi1/
wwwPath = /var/www/brewpi1/
port = /dev/brewpi1
altport = /dev/null
boardType = uno
$ sudo nano /home/brewpi/brewpi2/settings/config.cfg
scriptPath = /home/brewpi/brewpi2/
wwwPath = /var/www/brewpi2/
port = /dev/brewpi2
altport = /dev/null
boardType = uno
Hopefully something very close to this will work with Fermentrack...
Cheers!