My experience at DUS went very smoothly.
My flight was about 30 minutes late arriving so layover was cut down to 90 mins. LH was not able to issue a boarding pass for the TK flight at origin, so I needed to get a BP during my connection.
Making the connection from terminal A to terminal C involved clearing entry immigration upon arrival, exiting A and finding the TK desk to issue BP, clearing security to enter C and clearing exit immigration for my flight out of Schengen area. Was able to do all of this with probably 30 minutes to spare.
Entry immigration was very quick, exit immigration was a bit of a mess.
Not quite as easy as the all-LH connections I've had at DUS in the past, which can be done in about 5 minutes, but still very easy due to the compact size of the airport. Much preferable than making a similar non-Schengen connection at FRA where there is seemingly no rhyme or reason to anything.