Hogsmede is Islands of Adventure and Kings Cross is Universal Studios. A one day ticket will allow you to explore just the one set, but a multi-day ticket also allows you access to the Hogwarts Express, which goes between parks.
No extra charge, great experience
As sbove
You need the multiple park pass to ride Hogwarts Express between Hogsmeade and Diagonal Alley.
You have a multiple parks pass or is it just a multiple days pass for one park?
There is no extra charges to see both Harry Potter attractions!
As long as you have a park to park pass you can do all the Harry Potter things
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