In Disneyland Paris you must watch that the night you're staying, Disneyland is hosting fireworks. There was one time I went during a weekday last year, assuming it was like in WDW that there were fireworks every day...well, that wasn't the case that night as there were no fireworks scheduled. So just check that out to be sure, it might be a defining thing to the question "when to go".
As for squeezing the 2 parks, I have always squeezed the 2 parks in 1 day, even when I went for January 1st a couple of years ago. Disney's Studios in Paris is quite small right now, you have roughly 5 differential attractions from the American parks (Crush Coaster, Ratatouille, RC Racer, CineMagique and Armageddon). I would qualify it as a half-day park for sure (that is until they materialize the overhaul with Marvel Land, Star Wars Land and Frozen Land... THEN it will be a full-day park for sure).
Disneyland Paris itself, on the other hand, is very similar to the CA Disneyland, but with a couple of differences. The castle is beautiful, the Dragon's Lair bellow is awesome too, do not miss it. As for attractions, as I said, Space Mountain Mission 2 (or HyperSpace Mountain now) is a must; Indiana Jones roller-coaster also, as they are unique to Disneyland Paris. The Alice in Wonderland maze is great for kids. The Fireworks are great, using stuff similar to Happily Ever After in Orlando but less detailed and perfected to HEA as that one is brand new. They do use a combination of waterfountains that actually turn it out to be a great show. You could easily spend a full day in this pak.
In conclusion, if you want to be rushed but save 1 full day, squeeze them out. If you prefer relaxed, then you can take 1.5 days, or 2 days. Reserve enough time to stroll around Paris, too!
Hope this helped!