If you stay in one of the on site hotels you can easily do both parks in 1 day. Actually you can do both parks, go back to the hotel, swim, take a nap and eat in the park restaurants (you get preferential seating with your room key.), ..All between check in and check out. AND we went in peek season..middle of June. There were 4 adults and a 3 year old and everyone did everything as many times as they wanted.
You get speed passes for everyone staying in your room at no additional cost..it is your room key. It works like a credit card for park purchases and everything is charge to your room. All you need to take to the park is you room key. .You will have basically no wait for the water taxi to the parks so you can go back to the room when ever you need anything. Your pass is good for the day you check-in and the day you check-out. When you subtract the cost of speed passes from the room cost it really isn't as pricy as you think and don't forget parking is free for hotel guest.
You can check in early but you probably won't get your room until the regular check-in time. Early check in was hectic but don't worry you will have plenty of time for the park. It was easy to get in the park because we bought our park tickets before we got to the park.
I was leery about the speed pass so I went to an attraction that had a long line. We cut to the front through a second entrance. My husband was critical of speed pass saying it wasn't fair but I told him they had the same opportunity as we did. All they had to do was spend the money..your time or your money.
I'm not sure but I have been told the speed pass you buy at the park doesn't work exactly the same, but I don't know.
Some tell me the lines are so short off-season that you don't need a speed pass. As for me..I will stay on-site every time.
OOps This is for Orlando Unniversal..Don't have a clue about California.