I think you may be over-explaining. The poor guy just wants a cheap ticket into ATH and returning from IST and doesn't realize there are other ways to do it besides buying two one-way tickets. As AlphaGolf and I both said, just tick the multi-city button and you'll be fine. As for one-way tickets being consistently more expensive, I meant on international trips... because that's what the OP is asking about, not flights to Chicago.