I have several times seen pax in Berlin experiencing major difficulties at TXL checking in luggage on two separate TK PNRs. In fact, TK refused point blank saying they had received instructions from head office not to do so. Many Turkish expats know that domestic tickets purchased in YTL are usually extremely cheap and it is far more cost effective (especially during holiday periods) to combine two fares when flying from Germany. However, the cheapest TK fares have very restrictive rules such as a 15kg baggage allowance and this may be the reason for the problem.