Hey there!
A buddy of mine and I did this last year in August. We did it on a Thursday--so that helped with crowds, I think. I would expect it to be more difficult to do on a Weekend. Nevertheless, it IS possible!
Certainly it depends on what you want to do and what your priority attractions are, but here's what we did:
We started with the STUDIOS and arrived about 20 minutes before the 10:00am opening time. They open the gates before opening time, so you're able to be in the park, even though the rides aren't running yet. (We figured it best to start at Studios because Extra Magic Hours were happening at the
Disneyland Parc so, even if we got there at opening, there would still be other people in before us.) We got a Fastpass for Rockin' RollerCoaster and then headed straight over to Tower of Terror and waiting in line there when it opened. After that we went back and used Fastpass for Rockin' RC and (since the line was still short) actually waited in line to do that a second time. We then headed over to the area for Moteurs...Action stunt show and it was a good thing that we arrived there 30 mins before show time. It was packed! We then did CineMagique, grabbed a bite at the Studio 1 Commisary (not the greatest food...but it was convenient!) and headed to the main Parc as that's all we really wanted to do. (Had we known about Animagique, we could have easily gone to that after Cinemagique...but we were first timers...Studio Tram Tour was closed.)
At the Disneyland Parc, we headed straight for Fastpasses for Big Thunder Mtn Railroad, then worked our way around to major rides with short waits (Pirates, Small World, Space Mountain). We also used FastPass for Buzz later in the day. We had a quick bite to eat at Hakuna Matata counter service in Adventureland--we liked the food and enjoyed listening to Lion King music in background. Got to the main square in front of Castle about 45 mins before Dreams for a good spot, then had some drinks in the Village after the 8:30 park close.
We didn't get to do EVERYTHING, but we did get to do most of the major stuff and all of our priority attractions. It's quite doable, given the right circumstances!
Enjoy!