You can, most definately, experience the "essense" of both parks in a day...
Start your day at DL...
Ride Buzz Lightyear first thing
Get a fastpass for Indiana Jones
Ride Jungle Cruise
Ride Pirates
Get a fastpass for Splash Mountain
Ride Haunted Mountain
Ride Big Thunder Mountain
Head over to Fantasyland
Ride Mr. Toad
Ride Snow White or Pinocchio
Ride Small World
Get a FP for Space Mountain on your way out of the park
Go to California Adventure
Get a FP for Soarin'
Ride TSMM
Ride Screamin' Single Rider
Ride TOT Single Rider
Use your Soarin FP
Go Back to DL
See the evening parade
Use your Indy,Space, and Splash FPs
See the Fireworks and second Fantasmic.
If you still have energy, ride the other Fantasyland rides