So projected attendance last night was 41,000...during the summer (bearing in mind that Diagon was brand new), the average attendance was in the mid- to upper-30,000's. The crowds at HHN are getting ridiculous and I think Universal has to do something about it by next year. If they wont bring back the 2 park layout (which makes sense), then I think ticket prices have to increase. Im sure Universal can figure out how much they would lose in attendance and be sure to recoup that money in their ticket price increase. I mean honestly as it is now, the event isn't worth the price, considering its practically impossible to see everything depending on when you go. Sure, some people might get upset but in the long run, satisfaction will go up if the event were less busy.
I doubt a two park event will ever happen. There are so many x-factors that it just hardly seems possible, practical, or profitable.
Look at it this way, with a one park event they are able to keep IOA and DA open for regular day guests until 8pm without any HHN people intruding. These types of guests most likely came specifically for the parks and have zero interest in HHN so they are buying merchandise tailor made to their in park experience.
While HHN does good numbers, the most crowded nights recently have been ROF and FFP nights meaning that a lot of people have caught onto the value of those tickets. The type of ticket they buy is cheaper than a regular 3 day ticket so right up front you are not making as much money. HHN people are most likely buying HHN merchandise and not concerned about any other merchandise within the park as they most likely are AP holders or have already had day admission to UOR.
Running a two park even also ties their arms a bit as the next HHN year might not be as popular and they have to close BOTH PARKS early for the event. By having both open and running, they are raking in more money (albeit, at HHN guests and guests without parkhoppers expense).
If they do run a two park event, I would expect the prices to go up quite a bit to match the added attractions they would need to staff and to give themselves a bit of a cushion if the event is not as popular as this year's. After all, they are making a lot of money by having a packed studios and getting everyone else to spend the rest of their time in IOA, so you would have to recoup that loss somehow.
Another reason they would probably avoid a two park event is that companies can rent out the lands of IOA for corporate functions. Having a two park event at that point puts too much faith in the fact that the crowds WILL show up.
The only possible reason I could see them opening both parks is if DA and IOA were not busy during HHN this year. That reason alone seems like a bad idea to run a two park event since you are so reliant upon HHN being packed out, but it is a possible (thought not probable) reason for them to do a two park event.