It depends. Universal has, at minimum, a license to use Marvel characters in its parks. Depending on the agreement, it may prevent Disney from using Marvel characters in the parks.
Universal might also actually OWN the theme park rights outright, meaning that Disney would have to negotiate with Universal to use the characters in their own parks. It's tricky, I assume once the deal actually is completed more details will be released, but I wouldn't expect to see them at a Disney park anytime soon.
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