That's what we did last time (our last visit, though was before all the Harry Potter add ons), and it's what we are planning to do in December. One day is plenty for us for both parks. We are mostly going to see the Harry Potter stuff too. And although we think the Universal parks are really nice, we are bigger Disney / SeaWorld fans than Universal fans.
I will say, though, we are going the week before Christmas week and that has super low crowd projections. The crowd level is a 3 our of 10. What I did was get the touringplans.com add on subscription for the Universal parks for under $5 with discount code, Wizard, and played around with two custom plans (9 - 1 at Universal) and (2-6 at IOA) to see what I could reasonably get in based on their projections. I started with a copy of their adult touring plans and started subtracting stuff. There were lots of attractions we didn't want to do anyway from these plans, as we had done them once before and once was enough for us. That made it super easy for us to cut a bunch of stuff right off. And then there were some attractions we'd be happy to do again that weren't must dos for us in anyway, shape or form, so we took out a bunch of these to make the day work at a reasonably relaxing pace and leave time for the Harry Potter stuff old and new. If you aren't familiar with the attractions, the site has detailed descriptions of everything, an authors rating and comments about the attraction and reader rating by age group. This is very helpful for deciding what to do and what to skip. // I love the plan I ended up with and am really looking forward to our day there. // They were smart to put Harry Potter stuff in two places. We would have just gone to IOA even though there isn't really a full day of things for us to do based on our interests, but 9 - 2 would have been great. However, instead of $96 for one park we are forking over $135 for the one day park hopper to see all the Harry Potter stuff, and it looks like it will be a 9 - 6 day and a really fun one.
For attraction counts on this level 3 day, it looks like we can get in our seven favorite attractions (three being HP things we haven't done before) and one show we are interested in at Universal between 9 and 12:45, have lunch, and then do six things at IOA (two Harry Potter and our other four favorites) between 2 and 5:30. This will be a perfect day for us and let us do all our favorites and a few second tier things we like too. Also FYI, we have a full hour allocated time wise to London and Diagon Alley (Universal) and a full 45 minutes allocated to the Three Broomsticks area). This is the exploring timing recommended by the Unofficial Guide.