But then I thought about wanting to play a specific disc (like the second disc of New Order's Substance...)

Ah, yes, Substance is a tricky call, as they (mostly) arranged the A-sides onto disc 1 and the B-sides onto disc 2. I think this could well come under the umbrella of "two works in the same box" rather than "one work that happens to be more than eighty minutes long".

OT: Why O why O why don't New Order bring out a CD set of (a) all the stuff that's on the cassettes of Substance but not the CD (both tapes are 90 minutes), and (b) the brilliant twelve-inches from 1987-1990? They could call it Unsubstantiated...

Peter