Disneyland and DCA CAN be done in one day. I've done it. It's rushed, and you have to know WHERE to go in order to make it happen.
My BIGGEST tip of all is to go to the Matterhorn for your first thing to do in Disneyland (since DL is open a few hours before DCA). Matterhorn does not have a fast pass, and after the park has been open for an hour the lines become longer than an hour wait. There are TWO lines for the Matterhorn, they both wrap around each side of the Matterhorn, the most popular line is always the closest one which is part of Fantasyland. If that line is long, you can try the second line which is closest to Tomorrowland.
Second, go to Fantasyland if you feel like riding the classics of Walt Disney. Kids and parents are usually not all awake and the lines are the shortest they'll ever be except for after Fantasmic and to midnight.
If the kiddie classics don't float your boat, go to Adventureland and wait in line for Indiana Jones or Pirates of the Carribean (AS the lines are getting amazingly long because of the new addition of Captain Jack Sparrow). The lines in this area should be decent until the 45 minutes after opening has occurred. Get fastpasses for Space Mountain, the line never moves. Also, the Haunted Mansion line always moves quickly so don't waste a fastpass on that ride if the fastpass distributions are open.
Splash Mountain is closed until further notice, I'm not sure when you're planning on going, but don't make the mistake of going down to Critter Country when the only thing that is available is the Winnie the Pooh attraction.
In DCA
Make sure to get fastpasses at the Grizzly Run, the wait is not worth it for the ride. ALSO, the fastpass distributions are NOT connected with the other fastpass machines, thus, you can get a fastpass at the Grizzily Run and step over to Soarin' Over California and get a fastpass for that ride immediately. It's very convienient.
Hollywood Tower of Terror's line is not just the one outside, there is also a lower and upper division of waiting. Depending on the crowds, the wait can last up to an hour. Fastpassing would be a good idea unless you go right at DCA opening or at night when the crowds have moved back to DL, but Fantasmic is usally at 9:45 so planning is key.
Screamin' and Mailboomer lines move pretty quickly so fastpassing in my personal opinion isn't necessary. The Sunwheel lines NEVER moves, You'll be stuck there for a while if you choose to try the moving carts.
ALSO, Muhulland Madness DO NOT WAIT IN THE LINE. If there is only 3 of you and you want to try the ride ask for the Single Rider Line, you won't be in a group together but you'll be put on the ride with a group of 2 or 3 and fill up the seats. The ride is not worth waiting for, trust me. It's like the Animal Kingdom spin-round ride in the dinosaur ride, same basics.
All right, I think that's it hope the advice helps.