I’m using an external hdd holding two btrfs subvolumes. I added the disk to /etc/config/fstab two times each having the subvolume as their respective option.
It depends on when you need to mount points available. You can mount them manually through rc.local script. However this is executed as the last one so if you need the disk earlier than it might not work.
So I created my own startup script where I mount the subvolumes. it looks like:
Turris only mounts the last instance of the device from the /etc/config/fstab which is pretty anoying. If there only was a possibility to use standard /etc/fstab instead :).