How to use two ssd drives in conjunction

I replaced my old hdd with a new ssd about two years ago. I could only afford a 120 GB drive at the time, so as you can imagine, it really managed to fill up. Now, there’s only 9 GB available left on that original ssd. I purchased another ssd, this time a 250 GB (both drives are Electra 6G from owc) using the owc data doubler package which allows you to expand your storage by replacing the optical drive with an ssd. I would like to use both of these drives together to bring back some of the speed my MacBook Pro was missing. How can best use these two drives to increase my laptop speed?