As far as I know only Ryanair let you check in both flights 15 days before you travel .. and they don't fly to Turkey (yet). However you can only check in on line unless you want to pay another £60 to check in at the airport.
Having said that I don't recall having to check in again the last time we flew with FTC. We flew from the UK having checked in on line and then just turned up at Dalaman after ringing to check the flight was still on time before we left for the airport. Not sure if any of them still allow you to choose one or both options.