TK rules are the following:
*two different companies /two different tickets-bag is tagged ONLY to the point where is your first ticket is finished (your case only to IST);
*two different tickets but flying TK/TK and you have more than 2 hours connection time -bag (if you wish) can be checked to your final destination (but if connection is less then 2 hours -first case is applied ***)
*bur better to try to issue different companies in one ticket then your bag can be tagged to final destination and in case of delay of your first flight , your ticket can be changed free of charge , because of misconnection!