Does anyone know if you can purchase a Disney multi-day ticket without "park hopping" and use two of those admissions on the same day? Visiting Animal Kingdom and then going to Epcot on the same day, for example. I believe it's possible, but I'm not actually certain that you can do this. I can't figure out from the website (or my excellent Unofficial guide) if this is permissible or not. I think it must be a rarely asked question (in most circumstances this would not be a good way to do it).
I am going to try calling the Disney people, and see what they say, but I generally feel that you never, ever get the same answer twice from them, and the more out of the common the question is, the more varied the response you get. I would rather start here than Disboards, although I may resort to that.

If you buy a non-park hopping pass, then you will have to pay to use a second park on the same day. I don't whether WDW will accept another day from a multi-day pass. If you go to www.disboards.com (where there are far more Disney experts than here at Fodors) on the theme parks board, someone will know the answer to your question.

No, you definately cannot use two days on a multi-day pass to visit two parks in one day. The only way to visit more than one park in one day is to add the park-hopping feature, or purchase two separate passes. The most cost-effective option is to add park-hopping (I believe it is $50 per person regardless of the number of days).

Ok, I am obviously procrastinating taking down the Christmas decorations so I looked up the answer to your question. No, you may not use more than one day of a multi-day pass on a single day. In other words, you cannot park hop using a non-park hopping pass. The WDW computer system records your first entry and when you try to use it again on the same day for a different park, it denies your entry. WDW has this rule so that people don't use the less expensive multi-day tickets to park hop. You have to buy a park hopper pass or another single day ticket for that second park.

Three-day passes are park hopper tickets. You need one pass per individual...no matter if you're going to one park or two on any given day.
The reason why: it's not based on how many times you enter the park. You can enter one park multiple times on one day (say if you have lunch outside the park or go back to your hotel to rest).
Each person has to have a ticket.
The cost of the three day park hopper is the best deal...not only may you get tired of Disneyland but CA Adventure offers different shows and parades as well as my favorite - wine tasting!
Have fun!

The cost to "hop" from one park to another is $50 per person (in addition to the price of the ticket).

In other words, if you buy a three day base ticket and want to hop, it will cost an additional $50 per person. If you buy a 10 day base ticket and want to hop, it will cost an additional $50 per person.

As the others have said, you cannot use two admissions on the same day. You can, however, leave a park during the day and return later.

I believe Jello's answer applies to DisneyLAND in California, while the other replies, and the original question, are about Walt Disney World in Florida.

Check out the consolidator www.undercovertourist.com for small ticket discounts (including some free days on multi-day tickets), or if you are a member of AAA check their prices as well. They will save you a little bit over the gate price. The prices at Undercover Tourist include tax, while most others do not quote tax, so keep that in mind when comparing. I have never used Undercover Tourist, but they are highly recommended at MouseSavers.com and other Disney sites.

I recalled seeing something in print a couple of years ago to the effect that if you did not purchase park hopping, each park would be a separate admission if you visited it on the same day. Now I have not seen anything recently, and I think that this must have been around the time they debuted Magic your way and I suspect it had not yet occurred to them that a 4-day ticket was cheaper than a 3-day ticket with park hopping (and I think there may have been more difference between the 3, 4, and 5 day tickets). I have noticed that they have been really determined lately to structure pricing to encourage longer visits (and their exposure to your wallet). I'll probably double check with Disney on general principles, but I'm not too surprised that they've closed that particular loophole. Thanks, guys.