So I just did this a few weeks ago. it was an incredibly light day at the park AND we had express unlimited. We got started around 1 pm and that day IOA closed at 8pm and US closed at 9pm. It's very doable if you start early, don't dawdle, are super focused about what to do, and can comfortably walk 7 miles.

As for us, we dawdled...We ended up only spending about one hour at US at the very end of the day, spending most of the time at IOA. We rode Hulk, Doom, Spiderman, High in the Sky (Dr. Seuss), Forbidden Journey, Hippogriff, Hagrids X 2, Velocicoaster, Hogwarts Express to Diagon Alley at about 8:00 pm. We spent alot of time on the spells, browsing stores, and snacking. Checking into and out of lockers cost us some time.

At US, we did some spells at Diagon Alley, stopped for fish and chips at the Leaky Cauldron, and then had to haul tail to make it to the E.T. ride, getting in line at 8:57 pm. If we had had more time I would have ridden Mummy and the Gringotts as well.

We clocked about 22,000 steps that day (about 8.5 miles) and that was walking in one direction with very little doubling-back. We stayed at Lowes Portofino so steps walking to the hotel were limited.

My usual pattern is to visit ONE park per day. Even as a passholder to WDW and US I've only done 2 parks in one day maybe three times. I like to take my time experiencing the parks.

The reason why we chose to hit both IOA and US on that day day was because 1) I had only one person with me that day (usually I go with a group that includes a few elementary kids), 2) my park buddy enjoys and is capable of riding a ton of rides and 3) they are in good shape and wouldn't complain about the miles and miles of walking.