What are your tickets - IST-FRA RT and FRA-KIX RT? You are nesting your tickets (the KIX route is within the IST-FRA part) here, so risk LH cancelling your KIX route. They might pick it up at the start and deny you boarding for the FRA-KIX part, or you might find that your return from KIX is suddenly gone or repriced. Nesting is generally prohibited by most airlines and they tend to pick up on it.

If you absolutely need to nest return tickets, you need to avoid using the same airline at a minimum, or same alliance at maximum (or don't provide your FF number on one of the RT to avoid being flagged). So use TK for the IST-FRA (or some other airline) but you'll have to collect your baggage and check in separately to LH since you don't want your record to show a nested ticket.