I have a somewhat similar setup, except that I have a (much) smaller SSD and I have 2 spinning disks.
I put the OS (Slackware) on the SSD. On the smaller harddrive (500G) I put a swap partition (64G since I have 32G of RAM and I use the 2* algorithm!) and make the remainder a single partition which forms a volume group (vg0).
Within this volume group I have created logical volumes for /tmp, /var, /etc, /home, /opt and /VMs. This makes the SSD drive virtually read-only. By using LV's I can start with minimal size and expand as needed.
I have the second harddrive set up as a single volume group (vg1). I use the /VMs directory (mounted LV) to hold the virtual machine definitions. This directory looks like:-
/VMs ---> images
images holds any iso images or other stuff that the virtual machines might need (this is a convenience to me).
Each virtual machine definition is in it's own directory (vm1 etc). Each virtual machine has one or more logical volumes in vg1.
Virtualbox will allow you to use a disk partition (logical volume) directly as a virtual harddrive. This involves using the command-line tools (see virtualbox docs).
Or you can mount the LV and create virtual harddrives as files under this mount point. Using the second route, you can define the LV just big enough to hold your initial harddrives, and then expand it if (when) you want to add more virtual harddrives.
This is the theory!!! ....
The practice ....
Tell virtualbox that you want your machine defs to be in /VMs (or wherever)
Create virtual machine vm1 (using the virtualbox wizard) but do NOT create a virtual harddrive.
This will create the directory vm1 under /VMs which will contain the virtual machine definition.
Using route 2 above, define your LV, format it with your favourite file system, mount it on (say) /mnt and copy all the files in /VMs/vm1 onto it.
Erase all the files in /VMs/vm1, and mount your LV onto /VMs/vm1.
Now go back into virtualbox and edit the settings to add your virtual harddrives.
Thus you get all your virtual machine definition onto the one LV. NOTE if you define vm1 (and mount the LV onto it) BEFORE the initial creation of vm1, the wizard will barff because it needs to create the vm1
directory itself ... silly wizard but what can you do!
Repeat as needed.
This is how I do it. Others may (will almost certainly) have better ways and ....
.... as always, remember YMMV :-)