Greece-Turkey

What places are you planning to visit during your two weeks in Greece, if I may ask?

If I had the opportunity to extend a 2 weeks' stay in Greece by a couple of days, I would spend them in Greece...

Focus on Greece and save Istanbul for a trip to Turkey

The only really practical transport between is to fly, either starting or connecting in Athens, usually on Aegean/Olympic Air. Bus is long, train doesn’t go through, and ferries are both long and just connect the west coast of Turkey (not Istanbul) to the nearest Greek islands.

Is this part of your trip to France?

Alas, Greece and Turkey is one of those country combinations (like Spain and Portugal, or Italy and Croatia) that seem "natural" to Americans, but which are surprisingly difficult to combine on a single trip. As Laura says, the only practical way to connect Istanbul to Greece is by flying, most likely with a connection in Athens. That was true even pre-Covid; now I have no idea how often the flight connections run. Hence, the advice of the other posters to stay in Greece for this trip, and do a separate trip to Turkey that includes Istanbul.

I just finished a trip with both Istanbul and Greece worked out great for us. We flew Turkish air and had a “stop over” in Istanbul. Then flew to Athens and toured Greece. We used airport transfer in Istanbul which made it easy getting from and too the airport.
When we landed in Athens we took a connecting flight to santorini because we were already there. Then we flew back we rented a car from the airport and explored the Peloponnese.

I am traveling to Paris March 2022 and Greece Summer 2022.

Turkish Airlines often has a good price for flying to Athens (or many other cities outside Turkey) through Istanbul. This would make istanbul your “stopover” and Athens would be your “final destination”. We just did that last month (November 2021) and it was wonderful. If you can extend your stay for a week or minimum of 5 days, it would be worth it to go to Istanbul. If you can only add a day or two to your itinerary, you are better off staying in Greece. Istanbul is an amazing city to see and no one can do it justice in under 5 days. There are many other parts of Turkey that are excellent to see as well. Maybe do a separate trip to Turkey another time? See the RS Turkey in 13 days tour to see what the important historical sites are.

If you did visit Istanbul, you would need more than two days to do it justice. We did five days there and still didn't see everything.