Can confirm that I "smell" through one nostril at a time. I can smell more sensitively than others (for example, realise that milk is rotten before others do). I have noticed that when I try blocking one of my nostrils, my sense of smell is considerably lower, and it is not always the left nostril or always the right nostril. It kind of alternates, and I don't know the pattern. It might be that both nostrils alternate doing the task of smelling at different times.
If you do not have this behaviour (which can disappear with certain neurological diseases or just idiopathically not exist) you also likely have a reduced sense of smell.