Is it possible to visit both parks in one day without running between attractions?

Physically, yes. Not sure what you mean by "running between attractions". You can easily walk from one to another.

If you express pass on everythig it's possible, but you miss all the details in the theming.

It depends on how in depth you want to visit each park. I have taken school groups to both parks and there are some kids and parents who are finished with a park in about 4 hours. They've seen the big attractions, done the big rides....they don't care much for sitting through the shows on the Universal Studios side. On the other hand, I have some parents who could spend two days in one park and still not feel like they did it all. Personally, I could probably do both in one day, if I got there early and stayed until closing.....but it would be a long, hot day!

If you only have one day, and you are not likely to be in the area again for a long time, you can get through both parks. You will probably want to buy an express pass so that your wait is shorter on the big rides. If you have a choice though, I would recommend one day for each park

How much theme parking can you take in one day? Personally, I wouldn't do this, but you might not mind a rushed experience.

I've actually done it, but I know both parks pretty well and knew exactly what attractions I wanted to do. As Betsy says, it would be a long day (and yes, the Fast Pass is what makes it possible).