I happen to have two external drives connected to my iMac—one that reads and writes the usual mix of CDs, DVDs, etc., and another that includes Blu-Ray playback (but not writing). Today I discovered that you can use both drives at once (sort of) to speed up multiple CD rips. Here's how it works.
When you have two drives in iTunes, you'll see one CD icon in the iTunes 12 bozo bar—that's my name for the row of device controller buttons seen at right.
Click on the CD icon, though, and you'll see both inserted discs are available:
There really isn't a trick to using two drives at once in iTunes, other than saying "Yes" when this dialog appears onscreen:
iTunes won't actually rip both CDs at once, but it will queue the second CD up, and start ripping it automatically when the first one finishes. As soon as you see that changeover, click on the CD icon and switch to the just-imported CD.
Eject the just-imported CD, insert the next CD to rip, and say "Yes" again when iTunes asks if you'd like to import it. Repeat as necessary, until you're done. I imagine that if you had three CD drives, this would work just as well—I can't test that assumption, though.
While not fully automated batch ripping, this process does let you make relatively quick work of a stack of CDs—for those of you who (like me) still prefer such relics of a prior age.