You can easily do Disney Studios in a day
but probably not The
Disneyland Park as well in the same day.
I would spend 1 to 1 and a half days at The Disneyland Park and Half to One day at The Studios.
The studios is very much 'undeveloped' at the moment, with few attractions, although some excellent shows and a good movie parade. You can't miss Le Moteurs Action stunt show - it is fantastic and so is Rock 'n' Rollercoaster easily rivalling MGM's version. If you are hoping to do ALL of the shows and attractions within The Studios then you will probably require a full day, however if you are willing to miss a few than the major things can be done in half. The Studios don't get anywhere near as busy as The Disneyland Park (from my experience) and queue's are usually at a minimum, which helps to speed up the visit.
There are many attractions in The Disneyland Park which easily outshine their Florida and in some cases their DCA Counterparts these are 'don't miss' and include; Pirates of the Carribbean, It's a small world, Definitiely SPACE MOUNTAIN and Big Thunder Mountain.
The fastpass system is available on various attractions within both The Disneyland Park, and the studios.
Just make sure you get there for opening and be there for when the gates first open. This way you can whizz to all the popular attractions before the queue's start to build up at around 11am - 12 midday, and collect some fastpasses for early afternoon when queue's peak.
Altough it may seem like a lot of hassle, have you considered staying at a Disneyland Paris Resort Hotel for the last few days? They are beautifully themed and all within walking distance of the two parks, however if you are hoping to spend most of your vacation in Paris, then I guess a Paris hotel would be more suitable, as the train journey is cheap, reliable and fast to Marne-La-Valle.
Have a fantastic time, if you have any questions I'm sure someone will be a ble to help.
Richard.