What is the point of having evolved two nostrils?
There are two aspects to your question.  

First, organisms have some kind of symmetry. The field of evolutionary developmental biology has some theories on why our anatomy is the way that it is.  So, we have two eyes, two halves of our nose and two nostrils, one "tube" that becomes the path from our mouth to anus, etc. Our nostrils are just part of that symmetry. It may be evolutionary advantageous to keep two nostrils so that an animal can tell what direction a smell is coming from.

Second, we see that evolution has provided a path to having just one nostril.  Blowholes in toothed whales are just one nostril.  The other nostril does not have an opening to the outside of the whale.  It would be interesting to explore the split between baleen (two nostrils) and toothed whales (one nostril).  This evolutionary divergence may have been caused by differences in communication styles or the different blowholes may have caused a difference in communications http://www.whoi.edu/science/B/people/kamaral/marinemammalacoustics.html .