I don't think there is a need to divide the capacity in half. I don't believe the demand would be doubled as a result of a station in each park. Some people may ride it multiple times for that reason, but also some people will not ride it at all.
But by virtue of the fact you can only get on it travelling one direction, the actual line you are in for "the attraction" has a capacity that is half of the total. You really have to treat each direction as separate attractions that are separated by space and have separate queues.
Think of it another way: Duelling Dumbo's has one line that feeds directly into two separate spinners, but you are only getting on one. So the line you are waiting in now has a "double" capacity even though there are two physically separate Dumbos.
But before they made this move, if you were to throw another spinner halfway across the park as they did with Aladdin. It doesn't acutely change the Dumbo line, because they are different lines. Sure the total park-wide spinner capacity was doubled, and some people will only ride one, but many people will still choose to ride both because of their slightly differing nature. The total capacity and number of rides one can get on is now greater, but it really doesn't alleviate the Dumbo line apart from those select few that are happy just hopping on Aladdin instead.
The same can be applied to Hogwarts Express, even though there are "two" Hogwarts Express experiences, you are still only waiting on one, which is half of the total ride capacity and your line will only move at half of the total hourly capacity.
The problem (like the Aladdin spinner) is that in guests minds they are actually two separate rides, and while some may only ride HE once, there will still be a ton of people who choose to ride it twice, because they are "unique" attractions.
The second issue that makes this worse than my spinner analogy is that now you open it up to a larger pool of guests (all the people in IOA and Universal Studios), so that even if only 50% choose to ride it in both directions, your wait time heading in any given direction is artificially raised by guests from another park heading back in the other direction.
Really all I'm getting at from this is that ~2700 guests, or a ride capacity of 1350 per hour for a major
family E-ticket is no longer the wonderful ballyhooed guest capacity we once discussed. It's not a problem unless an unnatural number of guests entering the turnstiles HAVE to ride that ride as part of their day experience (or because it's the only ride like Soarin' or Toy Story Midway Mania). Unfortunately, this is Harry Potter we are talking about, and everyone is going to want to ride this thing over and over again.
Mark my words, this thing is going to have massive waits. They should have built it for three trains.
It's also going to be a frustrating line to wait in, because the line will only move forward once every full ride cycle.