I have a flight from the United States to Istanbul, and I will stay there for a couple days. Then I'll go to Venice from Istanbul and after 11 days I will leave Italy from Rome. My return flight from Rome to the US has a 2.5-hour layover in Istanbul.
If I leave my luggage with baggage custody at the Istanbul airport and check in one bag in Rome, can I check in another baggage, which will be in Istanbul airport, during the layover? Does the airline allow me to check another bag in Istanbul?
Doubtful. For several reasons.
First, you'd have to enter Turkey again (ie pass Immigration), pick up this extra piece of luggage, go to the check in counters, check it in, and go back to the transit area (security checks, immigration, go to gate, secondary security check). All that in two hours.
If you have one ticket, one one airline, Rome back to the US, via Istanbul (I suppose via Turkish airlines), your check-in is final in Rome, and you're not supposed to check in again along the way. Depending on the country, you might not even be allowed to leave the transit area.
I am not sure there is even a luggage storage facility in Istanbul, especially not after the terrorist attack there. You should check.
All in all, I doubt that this is possible. But you know what? Instead of asking here, you should talk to the airline. They have the knowledge and answer.
