I find that to be absolutely untrue.
If you go at a relaxed pace, you can easily finish both DAK and DHS in one day. DHS you will have to have the correct timing since show schedules can overlap. And you don't have to see everything...some things you may or may not be interested in, so you should never waste time on something you don't want to do just to say you did it.
Depending on the crowd levels, it is entirely possible to do almost everything. I'll give you an example. I arrived at DAK roughly around 10:30am. I headed straight to the Safari...to get a FP so I could catch Flights of Wonder. Along the way I planned to simply look at the animals around the Tree of Life...but most weren't out (there were signs saying as much in each exhibit). So I continued to Africa, got my FP, grabbed a pretzel and soda, and attended the "school" session offered by a CM from Africa. After which I went to Flights of Wonder and then headed to the Safari (all within an hour). I did the Pangani Forest Trail and then headed out to conservation station via the train. Headed over to Kali River Rapids, then the Maharajah Jungle Trek. Over to Everest, Dinosaur, Primeval Whirl and Bug's Life. I decided to skip The Lion King since I had seen it before, and Nemo, and went and rerode Kali and Everest. It still wasn't even 4:00pm. I could easily have fit Finding Nemo and Festival of The Lion King into my schedule but decided against it. Keep in mind, I did this all at a leisurely pace with no plan. It was cold out so no one was riding Kali River Rapids. I probably spent an hour on re-rides...Everest was also a walk-on. But even on days where it has been crowded I managed to do everything...if you utilize FP then you can finish everything.
Same with DHS, though it is a little more complicated. There are more attractions at DHS and timing is important, so just make a schedule of the shows you want to see, work the FPs for the rides you want to go on, and you will have no problems. I always get a park hopper, so when I am done at DAK or DHS I head over to Epcot. The only line I've ever had to wait in was for Soarin'. So if you take a full day at DAK, head over to Epcot when DAK closes, and hit up Future World. Do the stuff that closes at 7pm first...most of that stuff should be walk-ons by then. At any time, if available, get FPs for Soarin' and maybe Test Track. Then, if you spend a full day at DHS, go to Epcot again in the evening at hit up the World Showcase. I've found that if I go to DHS early enough (around 9:30am), I can be done with everything by 4pm...even on crowded days. So you should have plenty of time for World Showcase. That's three parks in two days.
Now, this is based on crowds when peak wait times are around 30-60 min. I don't know how it would work in the summer, since I have no desire to visit during the summer.