Sorry for that random post above... still learning how to use a forum...
Part of the confusion came when I was in the initial setup phase trying to link the 18i20 with the OctoPre.
Channel 1 on the OctoPre became Channel 11 in my overall rig. Later on, when I made additional input changes and SPDIF took Channel 11, OctoPre Channel 1 was actually 12...and so on. Just a difficult number offset for my already numbers-challenged mind.
I've attached a quick screenshot of one of the screens in Focusrite Control, just to give you an idea of how it is laid out if you aren't familiar.
It didn't help that I was also learning the deeper routing settings in Logic as well. It was just more advanced than I was used to.
I've got everything configured now though, and both units work flawlessly. I even use the 18i20 as my general soundcard when not working in Logic.
If you have any other questions, or if you end up getting it and have any feel free to ask!