a) long narrow noses are genetically associated with Nordic facial features.(Arslan A. Zaidi, Brooke C. Mattern, Peter Claes, Brian McEcoy, Cris Hughes, Mark D.Shriver 2017). This is associated with the fact that with a narrower nose, it is easier to warm up air, as compared to wide nostrils, due to low temperatures, both during travel and after landing.
b) On Mars, people will live in habitats. They will wear space suits and helmets while walking on Mars surface and they will be forced to live in air-conditioned spaces. Also the trip itself there will involve staying in air-conditioned rooms.
Already today, we spend plenty of time in air-conditioned rooms and this has its consequences (allergies, colds, dryness). Analogue astronauts practice such a preparation on Earth.
c) we cannot forget about mental factors. Long travel and time spent in closed space may impact anxiety or less severe symptoms such as discomfort. Relaxation techniques inspired by Pranayama breathing exercises show a huge impact of breathing through the nose on staying calm and providing oxygen to the brain. Double nostrils make i possible to strengthen the sense of a deeper breath.