I agree with Disney Dreams. You would not be able to see/do/experience
every single thing each park has to offer, but you can get a LOT done in one day. I would recommend it if both parks are of interest to you, if you only have one day to spend in them and if you are not certain when or if you may be back to DLR again. My friend from Atlanta had this very dilemma (a happy dilemma it is) back in July. She only had one day to devote to DLR while she was in town, and she was going to take her little son and her mom to DL only, not being sure when they would get back. I encouraged her to not miss DCA because I KNEW they would love certain rides and things that were there. So she took my advice and they did both parks in just one day, stayed overnight at one of the off-property hotels to save $$ and hit Goofy's Kitchen in the DLH the following morning, then went home. In any case, with some proper planning and guidelines, they got a LOT done in that one day in the parks. It was a very full, well-rounded trip, and they did almost everything they wanted to do with the exception of ToonTown (because on the way to TT, her son spotted Fantasyland and didn't want to let those rides go by!). They left for the hotel when DCA closed. In fact, if they had stayed until DL closed, they would have had even more time to do other things, but they LOVED Toy Story Mania so much that they had to ride it again before DCA closed up shop for the night.
So, yes! You can definitely do a 2-parks-in-one-day trip, and it will be very worth it. Getting to DL as early in the morning as possible and leaving as close to when they close at night would help, too, but even if you can't quite do that you can still get a lot accomplished!! Fast Passes for rides would help. If you decide to eat at a table service restaurant in one of the parks, then maybe make a Priority Seating plan beforehand to save waiting time in line. Those sorts of little tactics will get you a lot more time in the parks overall. Just make lists of the highlights you want to hit while at DLR!
