You cannot merge the two disks as they are, you would have to format them both entirely to configure them to a RAID. There is a process called tiering however, this is where you combine two drives with one drive being the 'fast' one (an ssd) which is used as the read and write drive which writes all the data to the 'slow' drive (an HDD). And thus you get SSD speeds on an HDD with the added HDD storage size.
But to reiterate there is nothing you can do at this stage in terms of combing your drives as is. You can control where your games are installed though, steam and uplay and other launchers always allow you to custom choose an installation folder, and you are indeed allowed to have more than one.