It's my understanding that it can't use two days because of the way the ATS ticketing system works. The computer looks at your admission in terms of "number of days", and whether or not your ticket has park hopping. Meaning - you've got 5 days of admission on a 5 day non-hopping ticket - NOT 5 admissions through a turnstile. Does that make sense?
Say on Monday, you go to Magic Kingdom in the morning. Then later on Monday, try to go to Epcot. When you put your ticket in, the computer can only say to itself "This ticket has been used today, but not here at Epcot. This ticket does not allow park-hopping." It then won't release the turnstile.