I installed PCLOS on a virtual machine and have been playing round with union mounting various virtual drives. I have succeeded with unionfs on the following (virtual) setup.
sda1 is mounted as "/" sda6 is mounted as "/home"
I created two additional 2 gig virtual drives, "sdb" and "sdc" respectively. "sdb1" is automatically mounted on boot at "/media/disk", "sdc1" is likewise mounted at "/media/disk-1".
The command "mount -t unionfs -o dirs=/media/disk=rw:/media/disk-1=rw unionfs /mnt/union" successfully creates a union mount of the 2 new drives on the directory "/mnt/union".
I can view the contents of both partitions "sdb1" and "sdc1" and writes to "/mnt/union" are directed to "sdb1".
While this is a start, it is not ideal yet, especially as having this set up caused the virtual machine to crash when shutting down. Also doing a little reading, unionfs has some problems that aufs has got round. I also need to figure out a way to make sure that on my AA1 the SD card is written to before the SSD (but only when it has more free space).
Like i said, it's a start. If anybody else has been experimenting, please feedback on what you've done and how it's worked.