NCT and ZOA not only conduct tours, but they actually encourage them. I went on a tour of NCT with the vZOA guys from VATSIM a couple years ago, and not only did we enjoy the tour, but they actually learned a few things from us as well. We also were able to get a tour of SMF Tower as well.
All you pretty much need is to give them a call and schedule it a few weeks in advance. They'll let you know when a good time is to have a tour, and you'll be good to go.
Keep in mind that ZOA is in Oakland, and NCT is in Sacramento, so unless you know of a good way through traffic into the bay, you'll be a few hours on the road, so I wouldn't expect both tours to be conducted the same day.