Orvieto and Civita di bagnoriggio

Orvieto is a small city.

CdB is a tiny speck with less than 20 inhabitants

You could probably see both in one day if you have a car, but you would not see all that much of Orvieto in a couple of hours...

Without a car, I have no idea

I'm assuming you'll have a car since you'll be outside of Orvieto. Going to both in one day would entail a lot of driving, with little quality time to experience either place. Orvieto is one of my favorite hill towns, and would make a wonderful day trip. I would focus on the Duomo, which is spectacular inside and out. Other sites are listed in the guidebook. Beautiful ceramics in Orvieto! Enjoy the 4 C's for which Orvieto is famous: Ceramics, Caves, Cathedral, Orvieto Classico vino.

Civita di Bagnoregio is a 40-min drive southwest of Orvieto, so getting there and back will eat up a lot of your day, which is why I would suggest going only to Orvieto for the day. I stayed in Orvieto for 3 nights, which made for an easy day trip (by car) to Civita di Bagnoregio. It is bittersweet to go there because it is a dying town (as Rick says, "it's like visiting a dying relative"), but it is geographically and geologically unique with the saddle road leading up to the town. Unlike anywhere I've been in Italy. There is not much to see there, honestly; so I would skip it. I would only recommend a visit if one is staying nearby and has an extra half-day to go.

If you plan to take the train to Orvieto, that would be even easier.

Don’t miss Orvieto’s Underground City.