We just did one day both parks, with no express pass. It was tight (and a long day!), but totally doable. We didn't get to do everything, but were able to hit all of both HP areas and everything else we wanted to see (crowds were not bad though).
One thing that saved a lot of time was that we wandered Diagon Alley and Hogsmeade looking in the shops during the day, but went back to Diagon Alley after close. The shops were closed, but the lights in the windows are all on and the kids did all of the interactive spells then. Only a few people there, no waiting to get their turn, and most importantly, they didn't feel stressed if they needed to do it a few times to get it to work. It seemed like it was much harder to get the sensors to read the wand movement when there were a million other people standing around waving their wands at the sane time, and they felt pressured to get it to "work" immediately so others could have a turn.
This also helped us so we could avoid the mad crush of people leaving right at 8.
I don't understand this after close thing. Does WWHP close before the rest of the park? Otherwise don't they want you to leave? I'd totally take my kids after close to do the spells if possible.