Originally Posted by
Pickles
You should be able to do both in one day, but you should start earlier as suggested here. Nijo shouldn't take that long (a couple of hours at most) but Arashiyama can take you days if you are into it, and you can easily spend many hours. Go to Nijo first, do the tour, etc. and then head out to Arashiyama, have lunch, and spend the rest of the day there.
One advantage of doing Nijo-jo as early in the day as possible is the absence of tour bus-loads of obaasans and ojiisans. The parking lot starts filling up with all the organized tours from about 9:00 or 9:30 am. If you can get there before that, you can have a nice, relaxing wander through the grounds, staying ahead of the first groups and be ready to leave after 10:00 am, just as the real hordes descend.