Yes of course you can. I have 7 in one of my computers.
If your C drive is full then you most likely have a lot of personal files (pictures/music/video) that you can then move to the other second drive.
You should also download and run CCleaner to clear out all of the junk system files, cached files, temp internet files and recycle bin.
You should also go through your downloads folder and clear out what you don't need.
Just doing thos last 2 things could clear anywhere from 4 to 30 GB from a system