I checked on the ticketmania website and it says:
*** One Day, One Park tickets to Magic Kingdom, Epcot, and MGM Studios , are not available for advance purchase, they must be purchased at the park the day you are going. Gate Price tax included is Adults ages 10 and up $53.00, and Kids Ages 3 - 9 $42.40.***
Not sure if this is gospel though.
You can also get 2 day, 1 park per day tickets ($105.43) and 3 day, 1 park per day tickets ($155.49) - you cannot park hop with these tickets.
Are you sure your only doing 2 WDW parks - which parks are you planning to miss?
Single day tickets work our expensive and don't give you the flexibility of park hoping. Hopping would enable you to spend the morning in one park, then hop over to another park in the afternoon to catch the eveing shows (Fantasmic, Illuminations, FITS & Spectromagic etc.)