I have to respectfully disagree with the folks who say it is impossible to do both places in one day, and I agree with ScooterL about the modified visits to each park. You not only CAN do both parks in one day, but my friend did it successfully last year - in the PEAK of summer - based on my suggestions and proper planning, and she threw in a character meal on top of it. They got a LOT done in that one day. The thing to realize is, you can't do everything. You are getting, basically, a 'highlight' tour of both parks and leaving certain things out, so you can have the best of both worlds. I never assume that everyone who goes to DLR will want to do every single thing there is to do in Disneyland because some folks just are not interested in certain things, and quite frankly, at times DL is so crowded that is is very handy to have DCA to escape to because it is less crowded. You can breathe and move around if you go into DCA during the busiest times of the day in DL. Surely, if you wanted to see every single thing there is to see in DL, you wouldn't be able to fit in a trip to DCA, but you wouldn't be able to fit in all of what there is in DL in one day, either. So since there is no way you can do ALL of Disneyland itself in one day, then you may as well pick the important things you want to do and then hop over to DCA for a few hours to do some things there, and then hop back over to DL to finish off your MUST-DO list. DCA has some very nice elements to it but you have to actually be in there to see them. And they serve alcohol, which may or may not be of importance to you. Many of the current sites and decor in DCA now will not be there in the next few years or will be dramatically altered, so it would be a good idea to see it in its current state.
I think it is well worth the money to get a one-day hopper.