Subscribing because we are trying to book a 11 day. I would try calling them before spliting. I am thinking (hoping) that they can do that on the phone. I don't see why not. People go for longer periods all the time.
We book everything ourselves. We use AP's and can usually secure an AP discount. If you want a package I would think, as others have said, that you will have to call direct to book it. HTH
We were there 15 days this past summer. I made reservations via the 800 number. We got 10 day park hopper with plus options which are good for up to 15 days of admission if you use the water park admission one per day (10 days main parks then 5 plusses on top of that). I couldn't do it online.