At Cedar Point on a Thursday if you arrive early enough, MAYBE. I don't think it is doable at Kings Island on a Saturday though. Kings Island can have some ridiculous crowds sometimes. I mean Cedar Point can be the same way, but for some reason it is more noticeable at KI. I have never personally covered all of CP in a single day, so you are pushing it there. Maybe it can be done on a Thursday, who knows. If it was me, I would plan two days at each park. If that is not feasible, ensure you get there before the place opens. That way you can get a jump on all the popular rides and then slow down as the day goes on.