Responding to the original post, I think the presentation of an album, whether it is one disc or two, the track order, etc., etc., are vital components of an album being perceived well. They are artistic decisions and are key to the listening experience -- at least for old school album folks.
As for the concept album 2 CD theory, the first that comes to mind immediately is Stone Sour's House of Gold and Bones. The first album was AMAZING to me. One of my favorite albums in recent memory. But the second half...it limps. Part of the problem is, they were released separately, so the lead track on the second album is more mid tempo, and the album sort of loses steam, IMO.
I think if you are going to release a concept, do it in one release, not two.
That said, even if it IS one release (on two discs) the pacing and story are key. If you don't have enough of a story to do a second part, make sure the first part leaves off at a spot where listeners are either left wondering what happened (in a good way -- purposeful speculation) or simply re-think the project and make it one album. Even for album people like myself, if something drones on and on forever, even with lengthier attention spans, it can get to be tedious. I think you always need to think about how a listener will consume the music. These days, that's probably on-the-go (in the car, or the stop-start of doing chores and errands with an iPod or similar thing.
Really good topic. But I think to dial it in:
1. Make sure there is enough captivating material to support a double-CD. Be liberal in cutting if you don't.
2. Be conscious of the pacing and the way your particular audience will listen.
Regarding Queensryche's The Warning -- that record went from being in the middle of the catalog for me as an album listening experience to my clear #2 behind Mindcrime...just by putting the track order in its originally intended sequence. Something just connected when I did that. I loved the songs I loved from it regardless, but that small tweak of the song order really made a huge difference. Both in my enjoyment of it, and artistically, as you can see what the band was trying to achieve with it.