We booked and stayed at the Fairfield at Sea World for 8 nights at end of Aug/beginning of Sept. It was a nice hotel (though my kids thought it wasn't as nice as the Portfino we had stayed at the 4 days before the Fairfield, and the pool didn't have a slide). Yes, If you book Discovery Cove through Sea World website, pick Sea World as the other park choice and you get unlimited visits to Sea World for 14 days from the first use. You can go to Sea World before or after Discovery Cove or both before and after. If you go to Discovery Cove first, your lanyard picture you get at Discovery Cove is also your ticket to Sea World.
Not sure how much time you have or how warm the weather would be, but through the Sea World website, we picked SeaWorld and Aquadica tickets with the hotel, and got a free day of dining at both parks. If you stay at a SeaWorld neighbor hotel like the Fairfield, all have a free shuttle to Sea Word (where you get a Discovery Cove bus to Discovery Cove) and Aquadica. Free parking and breakfast at the Fairfield, and best of all: While you have to walk over to the SpringHill Suites at the end of the one driveway (both hotels owned by Mariott), the specialist desk in the lobby gives you print outs for free unlimited front of the line passes for Sea World. For the three of us (2 Adults 1 Child) it was a savings of $126. It gives you front of the line for the coasters and free, unlimited access to the tower and boats.
We enjoyed Discovery Cove so much my kids think we will return next year. I told them the next time will be on their own dollar, it was expensive and there is no discount for kids!
I think you could do Sea World in one day, if not too crowded. But if you like shows, 2 days would be better. SeaWorld has many shows, so spreading them out over 2 days (which we spent) was about right. You also have to check the closing times, Sea World tends to extend days where it thinks it will have more traffic. For example, we went the last Sunday in August and they closed at 6pm. The Saturday of Labor Day week-end, it closed at 10pm. We did alot more on Saturday, saw alot more of the shows, including 3 that were offered Saturday night, but not offered the prior Sunday night.
Have fun, we did like Sea World, but kids (10 and 13) said ok if not get back there for a few years. The highlight was feeding the seals and dolphins and sharks!