I depends on what you want to know. Do want to know how many people are in the park at once? Have visited the park throughout the day? Are in the multi-park complex visiting more than one park? How about on party nights where there's a whole different guest list from the day to the night?
Here's something else to keep in mind: Let's say folks in Orlando have an AP to both WDW and Uni. They pop into Uni in the morning for their Potter fix. Then they hop over to Epcot for dinner and fireworks. Now, if we're only supposed to count park-hoppers just once for the day... which park gets to count them?
1. How many people are in each park at any given time? This impacts operations and tells you if it is safe to let more people in, if more crowd control measures are needed, whether to open the Main Street Bypass, whether to allow operations to extend staff members to increase capacity in the evening, etc.
2. How much money is being made by selling tickets? This is a function of the relationship between tickets sold and total money spent per person, how many hotel rooms were sold, was any discount required to make the sale, etc.
Note that annual attendance at each individual park doesn't really play into either one of these issues.
Disney has not been releasing annual attendance numbers since the '80s. However, OLC does release attendance from Tokyo Disneyland Resort (note that they do not have annual passes or park hopping for less than 3 or 4 day tickets).
The numbers that are usually thrown around come from the TEA AECOM Theme Index Report. This report is created primarily to create an industry benchmark, and should not be used for anything other than getting a ballpark estimate. In the book written by Buzz Price, the guy who first applied economic analysis to the amusement industry when Walt Disney hired his team to find a location for Disneyland, he described how they put together an estimated annual attendance number and ask Disney to check their work. Without violating SEC reporting guidelines, they have the researcher adjust the number up or down. But it is all unofficial.
I have discussed this with the author of the 2019 report, and she confirmed that this is still how it is done.
You can't count them as one when each park requires a separate ticket or a 2-park ticket to attend both. They're easily accessible, but clearly separate parks as defined by the turnstiles that you pass through to enter each one.
Indeed they are two separate parks. That is beyond clear: separate hours, individual park tickets, individual identities/themes.
I'm saying that I perceive them as one.....just a pet or personal viewpoint I hold since the Hog Express links the two lands of their respective parks seamlessly while fully understanding that they are bona-fide 2 parks.