if u r talking bout doing the thing where pay for 2 days and get 3 free, it is cheaper than buying a two day pass... so u may as well
then, spend as much time as you need in each park, and if you missed something or just watned to do it again, you could go back another day just for a little while...
this will be our first trip to US and IOA and it doesnt seem like we need very much time in comparison with disney, so we plan on doing a day at US, then a relax day, a day at IOA, maybe some time at city walk, or just go back to one of the parks one morning for things we really liked etc...
good luck!