Do you mean that, a 6D is not needed if we have a 80D? Because, my friend got an 80D(wildlife photography) and suggested me buying a FF like 6D(for landscape/night photography) so that we can swap based on our needs. I am a noob in photography.
Would want to see night images framed the same from both cameras to answer this one. The 6D is one of the best Canon has produced for night work.