For me, the closest Disney equivalent would be trying to do DHS and AK in one day. But DHS/AK have more shows and US/IOA lean more rides. Back 6 or 7 years ago when crowds weren't as big and before HP, Transformers, HRRR, at the like, doing most things in 1 day could be done fairly easily. Just like when AK and DHS first opened they were considered half day parks by many. As DHS added Sunset Blvd with ToT, RnR, and Fantasmic, and added TSMM. And AK has added Asia with KRR, EE, and Avatarland coming up. Trying to see all of boths parks in 1 day is becoming Impossible.
The same thing is happening at Universal. During slow season, 1 day is going to limit you to "highlights" only. Just like at Disney you could probably do KS, EE, Dinosaur, and 1 show at AK before heading over to DHS for ToT, RnR, Muppets, Star Tours etc. You're catching the highlight but you're still missing a lot. Same thing at Universal/IOA.
2 Days Park to Park would allow you to experience most things and a few of the favorites twice. For my next trip I'm looking 3 or 4 days so I can relax and don't need to rush around.
Having Express Passes will help. you do more is the lines are long as will the single rider lines mentioned above.