I have not tried this on the BPI-R2 but it should work.
As the SATA SSD will not seem to boot from uBoot I have used a modified '/etc/fstab' to support select directories
proc /proc proc defaults 0 0
/dev/mmcblk0p1 /boot/ vfat defaults 0 2
/dev/mmcblk0p2 / ext4 defaults,noatime 0 1
/dev/sda2 /sda2 ext4 defaults,rw 0 0
/sda2/bin /bin/ none defaults,bind 0 0
/sda2/sbin /sbin/ none defaults,bind 0 0
/sda2/tmp /tmp/ none defaults,bind 0 0
#/sda2/usr /usr/ none defaults,bind 0 0
/sda2/var /var/ none defaults,bind 0 0
/dev/sda3 none swap sw 0 0
# a swapfile is not a swap partition, no line here
# use dphys-swapfile swap[on|off] for that
It did not seem to work for /usr as /usr/dpkg/lock seemed to fail for some reason so thats commented out. But you should be able to add /home as well. Just partition /dev/sd2 as ext4 and copy the directories from sdcard to the SSD. and partition /dev/sda3 as swap, and run 'mkswap /dev/sda3', 'onswap /dev/sda3'.
/sda2/home /home/ none defaults,bind 0 0