You can pay to use one park only, but if you want to use both parks you pay a higher price, but it is not double - like $50 more. Go to the Universal website and you can see there are about 50 options to buy. 1 park 1 day, 2 parks 1 day, 2 parks 2 days, etc. And now there is the 3rd park - the water park - so like I said about 50 options...
They have it set up so you can do one or both
You only pay for the park you choose unless you decide to go to both
You can pay for one or both. You can also do the park hopper option that lets you go between parks more than once in a day.
You can pay for one or both, it's your choice. As the other person said if you want to ride the Hogwarts Express between the parks then you have to buy a ticket that allows you to go to both parks on the same day. Otherwise, you only need to buy one if you just want to go to the Island's of Adventure and don't care about the train.
You have to pay for both. And get a park to park ticket if you want to take the Hogwarts Express!
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