It does not work with another SSD. It is made for a Hard Drive system.
You could use it with a hard drive to accelerate another drive, but generally the OS drive is the one you want fast. So if you have an SSD, you will not gain anything
If you need fast secondary storage like a Photoshop scratch drive or a temp location for video editing, another SSD by it self is best.
By all rights, if you need a massive storage space for files, a hard drive should be fine.