Hey there, projectzdnb.
From what I understand you have no problem with the secondary SSD, but you want to use it's capacity along with the main drive without setting up RAID 0, but please correct me if I'm wrong?
Basically you won't be able to add the 2nd drive's capacity to the primary SSD without reformatting. That is, unless you decide to use 3rd party partitioning software. Either way you should backup all the important data you have before you try that out. My advice is to simply uninstall the game from your main drive and install it on the secondary SSD so that you don't have to worry about any future updates, DLCs, expansions etc.
Hope that helps.
Boogieman_WD