is 2 weeks enough time to see southern Italy and Athens?

It's just a personal preference, some feel like they should see a lot once they have already paid for a transatlantic flight, others would not want to waste time in transit that could be spent on the ground.
So, I won't tell you what to do, but just what I would do--one or the other. Perhaps consider including the Greek temples at Paestum, a but south of Naples--more preserved than any I saw in Greece (which I also loved) or exploring a bit of Puglia, "Magna Graecia."
A two-week itinerary that includes Rome, Florence, and Naples + surroundings would be perfect, anything else would be stretching yourself thin.

Florence, Rome, Naples, Athens and a Greek island or two, in '14 days"

Is this reasonable?

If 'see' is taken in a limited literal way, yes. ( e.g., I want to see St Peter's Basilica--- from the plaza you can do it in less than 5 minutes)

If 'see' means to visit in a meaningful way, NO. ( e.g. go inside St Peter's and view all the art and architecture will take hours)

Which kind of 'see' do you have in mind?

We like old ruins, art, churches, museums, and eating good food. Last year we did Milan, Verona, Vincenza and Venice in 10 days and it was wonderful. We didn't see everything but we saw ALOT. This year we thought we would extend our trip to 14 days. We saw alot in those days and only stayed in 3 hotels. Since its our first time in these places, we focus on the "first timer" highlights, art/castles and ruins. I had hoped that we could see a bit of Rome in several days and then do day trips from there to see Florence, Pompei, etc. It's pretty easy and fast to fly from Rome to Greece. Rome is the airport we will fly into roundtrip from Maryland. I plan this myself so its a bit overwhelming.

What about maybe flying into the city you get the best fare/connections/schedule in, be it Rome, Bari or Naples? Land in one of them, depart from another, with trains in between. Pack light. There are parts of Puglia which are quite Greek in origin and your experiences will be diverse for such a condensed visit to Italy. If you want an island experience, take the ferry to Ischia from Naples, day trip or stay on the island instead of in Naples proper. 3 areas in a single country, in my opinion, makes for an interesting, but not exhausting, vacation.

Sure you can do that! Have a great trip! The RS guide lists the 3 triangle (don't miss things) and if you want to get them in multiple cities (while skipping 2 triangle or less) and don't mind the travel-- trains and/or flights-- it can work great!

Last year we did Milan, Verona, Vincenza and Venice in 10 days and it
was wonderful. We didn't see everything but we saw ALOT.

To me this answers your own question. If you enjoy that style then do it. Maybe when you're back you'll decide it was a mistake, then it's a lesson learned and you can make adjustments for your next trip. Personally, we spent 14 days in Sorrento last year and that was perfect.... for us. But I bet if I were to post my itinerary as a suggestion of a future trip, l would get plenty of people saying that that is too long for the region, and it may be... for them. Go with your gut.

The ideal bare minimum would be
Rome 4 nights
Florence 3 nights
Naples 3 nights
Athens 3 nights
If you want to add day trips to the surrounding areas, then add a night for a location for each day trip. So for example if you want to take a day trip to Siena or some other Tuscan destination from Florence, add one night to Florence.

I think 4 cities in 14 nights is reasonable but I personally hesitate on spending an entire day effectively on airline travel for one of those destinations. Naples airport is close to the city and the flight only about 2 hours and then probably another hour before you're in the city so 3.5 to 4 hours without counting getting to the airport early. I can't help thinking that the airline travel days are almost completely spent on travel and there are 2 flight days out of your 14 days. And now you're in another country and probably need to get back to Italy to fly home.

I would recommend spending all 14 days in Italy concentrate on spending time there and not adding an entire other country to the agenda. Italy will satisfy all of your museum, church and ruins requirements for about as much time as you can through at it. Extend your time and make sure you have time for Ostia Antica outside Rome, Pompeii, Herculanum and Paestum along with the museums in the Naples area. Add a Siena or Pisa day trip to your time in Florence along with all the other Florentine attractions.

Just my opinion: Can you? Yes. Should you? I would vote No.

Whatever you decide - have a great trip!
=Tod

Thanks for all the responses. Definitely want to include Greece. Now my dilemma is round trip fares are so much more reasonable than one way. If I fly into Rome and return via Athens, the price is more than doubled. Perhaps I should return to Rome and spend a few more days there before going home. Also, never flown Emirates airlines which is significantly cheaper from Athens than United - any thoughts on that one?

Save Greece when you will have more time. There is more to Greece than Athens.

If I fly into Rome and return via Athens, the price is more than doubled.

Are you looking at one way tickets or 'multi city' ( aka open jaw) bookings?

one way and multi city - same doubled prices or more.