SyracuseWolvrine said:As a general rule, the CMs on the phone reservation lines know nothing about how the turnstyles work, and the CMs working the turnstyles know nothing about how the dining reservations system works.
The answer to your question, no you cannot use multiple days worth of admission on one day. If you want to visit multiple parks in the same day, you MUST have the park hopper option. (Unless one is a major park, and one is a water park, but that's an entirely different question). The turnstyle software will reject a non-hopper ticket that tries to enter another park. I used to work turnstyles, and still do from time to time.
If you don't mind my asking, what did you mean about the water park? Is there a way to use an extra day ticket for a partial day at a water park after visiting a theme park on the same day?