I think what you are referring to is when you buy your ticket for the following day the same day. So if you're at the magic kingdom & you buy that day's ticket and then buy the next day's ticket for Epcot at the same time (or at least before the MK closes for the day) you get $5 off the next day. I believe that these have to be puchased at the gate.
Here is the info from allearsnet.com (go under "planning" & then "tickets"):
TICKET MEDIA AND PRICES
For a complete list of current admission prices, please see our Ticket Price Comparison page. All prices quoted are for purchases made while at the WDW Resort. Advance purchase discounts are available on some ticket media. See our Ticketchart page.
One-day/One-park ticket -- Provides one admission to your choice of the Magic Kingdom Park, Epcot, the Disney-MGM Studios, or Disney's Animal Kingdom Theme Park. Valid for admission to one theme park only. $58.31 for adults (includes tax) and $46.60 for children ages 3 to 9 (includes tax) (3/04) This ticket does not allow park hopping.
2 Day/2 Park OR 3 Day/3 Park ticket - These are the tickets that were originally known as "bounce back" tickets when they first came out. You can only purchase the 2 day and 3 day passes at a WDW theme park ticket window, any Guest Relations location, any Guest Services location or at any Disney Resort front desk. You can not purchase them over the phone, at a
Disney store, at any Disney off property ticket source, or online. These tickets allow entry to only ONE PARK per day. They do not need to be used at the same park both days. You cannot park hop with these tickets but you can leave and return to the same park the same day. Be sure to get a hand stamp before leaving.
2 Day/2 Park ticket - Adults $111.30 (includes tax) child, $87.86 (includes tax) expires 4 days from *purchase*. (3/04) You save $5 off the price of two single day tickets.
3 Day/3 Park ticket - Adults $164.28 (includes tax) Child, $129.13 (includes tax) expires 6 days from *purchase*. (3/04) You save $10 off the price of three single day tickets.
NOTE: Second and Third Day options MUST be bought *at the time of purchase of the first day* OR *before the end of the business day of the date of the first day's purchase* --- NO EXCEPTIONS. If you come back a second day and say you want to upgrade, you cannot. You will be buying a new One Day Ticket at Full Price. There are no discounts on these tickets other than stated above.
NOTE: Guests will be limited to a purchase of SIX One Day/One Park Tickets at a time. (1/01)
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