There's a good reason why ears and eyes come in pairs.
Receiving stimuli from two points of input is necessary to pinpoint the location of objects. Most animals, including people, can hear and see in stereo. You can tell where a sound is coming from by comparing the time it arrives at each ear, and stereo vision allows you to see in three dimensions.
Nostrils also tend to come in pairs, but there are very few studies on whether the olfactory system can operate stereoscopically. A stereoscopic sense of smell would enable an animal to locate the source of a smell based on the strength or the timing of odors arriving at each nostril. Each nostril would send a slightly different signal to the brain, where the differences in timing or intensity would be translated into the direction of the odor.
Researchers found sharks are able to compare the timing of odors arriving at each of their nostrils to follow a scent trail to its source. As for mammals, lab rats can be trained to tell whether an odor comes from the right or left, but such behavior has not been studied in any mammals behaving naturally.
The eastern mole is a sturdy little burrower, highly specialized for living underground. It has tiny eyes, useful only for distinguishing between light and dark, hidden below folds of fur and skin. Its large, hairless forefeet are shaped like spades — ideal for digging. The most important feature for this study is the mole's snout: a fleshy, moveable protuberance with two closely-spaced nostrils on the upper part.
Catania at first did not think stereoscopic smell was possible in the moles, partly because of the close position of the two nostrils. Stereo sensory systems depend on the animal's brain computing differences (for instance, in timing or intensity) of the input at two points.
To test the moles, Catania designed a circular arena with 15 food wells spaced around a 180-degree circle. He placed pieces of chopped earthworm in one of the wells. Then he temporarily sealed the chamber so he could detect each time the mole sniffed by the change in air pressure. This also eliminated airflow that might disturb the flow of odors and simulated the closed environment of the mole's natural underground burrow. Catania monitored the sniffs and filmed the moles' search behaviors with a high-speed camera (see video).
First, for the control condition, Catania recorded the moles' behavior in the chamber. When a mole entered the chamber, it would move its nose back and forth while sniffing. The moles found the well containing the earthworm pieces in five seconds or less and went straight for the correct location every time.
Next, Catania blocked one of the moles' nostrils with a small plastic tube. When the left nostril was blocked, the moles consistently veered off to the right, and when the right nostril was blocked, they veered to the left. The moles seemed to be "pulled" toward the open nostril, even if it was away from the direction of the food. Eventually the moles found the food, but it took much longer than in the control condition.
Moles benefit from the ability to sniff in stereo. By simultaneously sampling the odors reaching each nostril, and comparing the intensities, moles can quickly determine the direction of a predator. Their stereoscopic sense of smell is also useful for tracking the scent of food to its source, especially in their underground environment.
Reference: Catania, K. C. 2013. Stereo and serial sniffing guide navigation to an odour source in a mammal. Nature Communications 4: Article number 1441. doi:10.1038/ncomms2444