Thermal design for an IGBT
You have to do it separately because IGBT and diode will have different junction temperatures. In a first approximation you can assume that the heat sink temperature for both devices is the same. 

dT,ibgt,jc = 50W * 0.4K/W = 20K

dT,diode,jc = 10 * 1.124W = 11.2K