From my personal experience you need a down day in there so that gives you 5 full days to do the parks and either way you will have more than enough to do..
I guess it depends what you are into more, Universal has better attractions overall (some say Spiderman is best attraction in the world!) while Disney has much better shows/fireworks and a more immersive atmosphere..Your basically doing a commando approach and everyone I ever talked to that did that regretted it..I know Ive done it..by day 4 or 5 you hit a wall and the wonder fades rapidly.
If you were to ask anyone that has ever been to Disney that you plan to do all 4 parks in 2 days they will tell you that your'e mad...I've been on 6 day trips to WDW and did not even do 50% of the stuff there is to do..
I'm starting to agree with you now that I've mapped this all out
Discovery Cove seems to be a "down" day since most of it is hanging out in the water and not too many attractions/activities to see...maybe i should put that in the middle of the trip.
Updated:
S - Travel (arrive 930pm)
M - Magic Kingdom and Animal Kingdom
T - Universal Islands of Adventure
W - Disney Hollywood Studios
Th - Discovery Cove
F - Universal Studios / city walk at night
Sat - Epcot
Sun - Travel (1030am flight)