I don't think you need a boarding pass to go through the transfer desk. At least that was my experience when I went through. I just had a print out of the email confirmation for my next flight. I showed that to the transfer desk and they let me through.

With this scenario however, you will need to secure the boarding pass once you make it up to the Departure level. Just note that since Turkish Airlines is part of the Star Alliance, you may have to go to the Asiana Transfer Desk once you go past Security.