Planning trip to Brazil/Argentina in December
 

I'm planning a 2 week trip in late december to Brazil and Argentina. I'm considering rio de janeiro for 5 days followed by iguazu falls 2 days. I would then fly into AEP and visit Buenos Aires for 5 days. Lastly I'd fly to Mendoza for 2days and fly back to ATL from SCL.

I'm wondering if my itinerary is doable or if I might have overlooked something. Is there a city I should spend fewer/more days in?

That seems like a pretty good itinerary.
If you can go thru to NYE,
you could reverse it end end up in Rio for
the amazing fireworks on Copacabana.

I tried hard to reverse the itinerary and fly out of rio after new years, but couldnt manage with miles.

I agree that the itinerary seems solid. Five days in Rio is plenty, and you could also do a day trip to Niterói or even some beaches near the city.

If you plan to visit the Brazilian side of Iguazu Falls (which I absolutely recommend), check out the bird sanctuary. It's definitely worth a couple of hours.

For BA, since you'll have a pretty long stay there, you might consider taking a day trip to Tigre and the Delta region (or even to Colonia in Uruguay. taking the Buquebús ferry).

Enjoy the trip!

After some more research, would anyone advise checking out glacier in El Calafate as well? I could cut out a day in Rio or BA and try to fly to FTE from AEP for 2nights.

No, simply because you only have full day in the region. I would plan on spending at least three full days in the region and I would definitely visit Torre del Paine in Chile too if you are in El Calafate.

If you can make your trip longer there are many interesting places to explore, if two weeks is all you have if you try to visit more places you won't enjoy yourself and you won't see the places you are going to properly.

El Calafate and Torres del Paine seems a lot even for three days. There is quite a lot of driving in these parts.
For me, 5 days Rio and 5 days BA is a lot, you can cut a day from both and visit the glacier. But that's just me, I prefer nature than cities.