Doing it in a full day normally requires the park to be open for a full schedule (usually 9am til midnight) and you'll probably have to skip the parades and/or fireworks if it's a busy day.
If you have some flexability in your schedule, 1 1/2 at DL and 1/2 at DCA would allow for a more pleasant visit.
All of this is predicated on when you're visiting. Trying to do everything in a day on Memorial Day weekend is probably impossible, while the first Wednesday in June would probably work. Also it depends on how you define everything. If you mean every ride from Alice & Dumbo to Splash & Space and all of the shows you've got a alot of ground to cover. If you just mean what you consider the good stuff (thrill rides or show rides) then it's an easier task.
Give us a little more detail and you'll get some more opinions.