Civita

It's 50 miles, and GoogleMaps says it takes 1 hr. 20 minutes to get there.
Sure you can take in Orvieto and Civita in one day.

Civita Di Bagnoregio is tiny. If you have a car, you can easily drive to Civita. If you are taking public transportation, then you should take a train to Orvieto. Spend the morning there for a few hours. Then, take the 12:45 pm bus to Civita. This will give you the whole afternoon in Civita which is ample time unless you want to eat a meal there. The last bus back to Orvieto is around 5 - 5:30 pm. You need about 30 minutes to walk from the bus stop to Civita. From the bus stop, walk up hill to the "historic" part of Bagnoregio. Get on the Main Street and walk away from the bus area until main street ends. You will be able to see tiny Civita. Before you cross the pedestrian bridge - you pay a small toll at a ticket booth.

Orvieto is worth however much time you can give it. I'd make it as full of a day as you can between the two because they're both great.

Orvieto is much larger and has much more to see than Civita di Bagnoregio.

Getting to Civita requires a great deal of effort for very little return. If you go on Trip Advisor and look at Thing to Do in. Orvieto, you will see that there is more than you can possibly cover in a day, let alone half a day. If you are searching for other places for day tripping from Montepulciano, check out Pitigliano, Pienza, Cortona, Montalcino, Arezzo, etc. There are loads of towns much more interesting than C di B.

I have only seen Civita dB from afar, in all my time in Italy. From what I understand, it's mostly for tourists on daytrips, no real residential life anymore.

You do not need to spend a long time there but it is very much worth a visit. The bridge is ugly, the view is nice, the village a lovely pile of old stone. Head there first, followed by Orvieto. We did it the other way around and by the time we reached Civita my wife had reached her fill of walking for the day, and I ended up crossing the bridge on my own.

Civita is definitely worth seeing and worth the walk. I did not find it ugly at all; I found it charming with fantastic vistas. That said if walking is an issue it is not the place to go. I would go there as early as possible to avoid tours. We had the place to ourselves as 10 AM last month. Orvieto really needs more than half a day. The Duomo is worth seeing though it won't take long and underground tour was excellent.

Civita is definitely worth seeing and worth the walk. I did not find it ugly at all; I found it charming with fantastic vistas. That said if walking is an issue it is not the place to go. I would go there as early as possible to avoid tours. We had the place to ourselves as 10 AM last month. Orvieto really needs more than half a day. The Duomo is worth seeing though it won't take long and underground tour was excellent.