I would just reinstall the system and reconfigure the applications myself.
Keep your system files on 1 disk, so the SSD. 20Gb is enough to install all the software you will want to install (well not if you install everything in the repositories ;) ). Otherwise get a bigger SSD.
I myself did the following:
/ on the SSD in a 20Gb partition
/home on the SSD in a 5Gb partition. I do not use this for my own files only to store the hidden directories the system itself installs. (I have a 120 Gb SSD and the remainder is kept unallocated).
/discworld on my HDD.
~./config/users-dirs.dirs to use
/discsworld so all my directories from /home are there. Basically all my personal data is on the HDD.
Yes, you can split the system into 2; it used to be normal to mount all the different root-directories onto their own partition on servers.
If you create a mountpoint called
/var on the HDD the contents of /var will be stored on the HDD. Have a look at the filesystem hierarchy for a list and explanation...
bin Essential command binaries
boot Static files of the boot loader
dev Device files
etc Host-specific system configuration
lib Essential shared libraries and kernel modules
media Mount point for removeable media
mnt Mount point for mounting a filesystem temporarily
opt Add-on application software packages
sbin Essential system binaries
srv Data for services provided by this system
tmp Temporary files
usr Secondary hierarchy
var Variable data
But ... if either your SDD or HDD breaks you can't boot your system. Plus it is a lot more work when you want to re-install.