While this has been discussed in other threads here at MiceChat, I have just confirmed the info, and wanted to make the new rule clear to MiceChat users.
Unlike past years, where the turnstyle CM's would allow the use of both "days" in a single day to visit both parks, Disney has changed its policy.
Let me quote from a SoCal Resident 2fer.....
Expires 30 days from first visit
Valid 1/3/06 thru 4/27/06
Valid for 1 Admission to Disneyland Park & 1 Admission to Disney's California Adventure.
Not Valid at Both Parks on the Same Day.
Please Present Valid Photo ID with Ticket for Admission
Multiple people have reported they have been sent to the Ticket Windows to upgrade to a SoCal 2 Day ParkHopper for $20 more when they tried to enter DCA on the same day.
So, FYI, be prepared to get the SoCal 2 Day ParkHopper for $79 if you want to visit both parks on the same day.