An SSD cache drive is different than a regular SSD. Since you have a regular SSD, you should just use it as you would a regular drive. A cache drive is a cheap alternative to a SSD. Basically, it only stores the data that you access most frequently. You cannot choose what goes on it, a program that comes with the cache drive chooses it. Your regulard spin HDD is still the main drive and you save everything to it. The cache drive works alongside it to speed it up.
You could buy a program that will use your 128 GB SSD as a cache drive. But it is large enough that you might as well use it as a regular drive. Just install the programs that you use most often on it and the OS.
There is also a different tech called Readyboost found in Windows. It uses a USB drive as a cache drive to boost the performance of your system. Having a SSD which is exponentially faster than any USB 3.0 flash drive makes it unnecessary.