why exactly do you need to transit in IST?
There should be cheaper fares that avoid IST.
Give us your dates and maybe we can help.

By the way, for your request to actually work, minimum of two conditions need to be met. How to find out, you either do the legwork on matrix.com, or talk to a real live travel agent.
1) both fares must have EOE permitted (end-on-end permitted). Otherwise, you cant tack on a QR segment to the TK segment on one ticket, and vice versa!

2) you need a plating carrier (the airline that issues the ticket) . It could be either of TK or QR (or a 3rd airline), but one of the fares must allow for the ticket to be issued on another carrier. i.e. segments on QR fare, issued on a TK ticket (235-xxx) or segments on TK fare, issued on a QR ticket (157-xxx)