I wouldn’t recommend it. JBOD’s disk sets don’t have any protections. So if either drive has a problem, both drives data is not accessible and useless! As laptops bounce around you are at a higher risk of loosing your data.
JBOD disk sets are useful for data sets that exceed a single drives storage space. Like a large image data base or video’s which make up a movie while it’s being edited. Even still these are more often placed on external RAID drives (RAID-5 or RAID-6). With RAID you need to have the same size & kind of drives and they need to use the same I/O.
JBOD doesn’t care about size or kind or even the I/O. But, you’ll pay for it! The storage of data is random! So you’ll see spits and spirts within your data access across the drives.
The better direction is just get a much larger drive and if you can swing it get a SSD.
As far as your system using the optical drive to host a drive …
You’ll want to review this page: OWC Data Doubler to see what your system is able to support for drive. Apple has a problem within some of its systems so even though the system appears to support a faster I/O SATA III (6.0 Gb/s) it can’t really do it. In these systems you need to use a fixed SATA II (3.0 Gb/s) drive (one that only runs at SATA II).