some years ago my sons and i were super excited when IOA first opened.
we loved the US side and wanted to hit the highlights in both parks.
it was not an easy task but by only doing the highlights and no shows, we did both parks in one day. we started out by being in line at 8:15, gate opened early, ate at 11 an off time and stayed until closing. then ate dinner off site.
yes, it can be done but it is NOT an enjoyable way to tour the parks.
we missed many of the shows and did not have time to look around and enjoy the ambiance of the themes.
no way would i try that again.
2 parks in 1 day is not ideal. if you buy the EPP it is for one time only per ride/attraction, no repeats. to repeat means to stand in the regular line unless you do the single rider line if available.
we were at universal for a 3 day stay and was off site on that trip. i guess we were so charged up to be there we wanted it all on that first day in the parks. the next two days we went at a slower pace and still did not get to see everything. the shows, sinbad, poisiden, horro make up, t2, animal planet, beetlejuice were all at scheduled times. they were real down times as the wait in line prior to the show, then the actual show cut into our time at the parks.
after that trip, we started staying on site. this allowed us to enjoy the parks without the long waits in line.
the longest we have stayed on site has been 6 days. i think we filled it up well. we took more breaks, did the movie and meal deal, shopped at city walk, left the park at lunch time to eat at city walk, etc.
we spent 2 days each at both parks. we would hop to the other sometime during the day in order to catch scheduled shows.
we enjoyed taking our time eating at city walk and had some down time going to the cineplex. 6 days was fine for us.
now i go solo to universal and cut it back to 4 days on some occassions 5.
i am amazed when people only plan to go one day to universal
the first time you go to any theme park takes adjusting. even with a map of the layout, you still can get confused and turned around. you can't get the feel of the park in the first couple of hours. it can get overwhelming.
i would skip the water park and spend the 2 days doing universal while you are staying at the disney resort.
water parks are everywhere, universal is not.