I ended up buying the the 14tb drive rchunter recommended. Fast or regular format?
I'm not technically sophisticated. I don't know how to combine two smaller drives into one. I tried using external servers to hold my ever increasing iTunes library, but didn't like it. It would take some time to mount, sometimes wouldn't mount, and I had to keep it on all the time when I use it only about once a month.
Putting it on one giant HD seemed to be the easiest solution. I'm not as concerned about backups, because I bought everything from iTunes and can redownload what I need.