If we only have one trachea, why do we have two nostrils?

Two eyes, two ears, two nostrils. We need our doubles for stereoscopic vision, stereo sound, and super smelling. Our nostrils are separated by a septum, in effect giving us two noses. Most of the time, one nostril allows less air to pass through than the other, with the nasal flow switching every few hours. The slower airflow is caused by the tissue inside swelling with increased blood flow. We smell using sensory cells high up in the nose, and some odour chemicals need more time than others to bind to these receptors. So a lowairflow nostril gives slow-acting odours more time to be detected, giving us a greater range of smell.