Question: Do I Need Two Pairs of Running Shoes?
I've heard that it's best to alternate two pairs of running shoes. Is that correct? Or is the salesperson just trying to get me to buy two pairs of running shoes?

Answer: If you run almost every day, it is beneficial to give your shoes a rest in between runs. Your shoes will last longer when you give them a day or two to decompress and dry out between workouts. This is especially important if you did a run in the rain or snow. Wearing running shoes that are fresher will help prevent injuries more than if you ran in the same ones every day. The extra cushioning in the shoes will help protect your joints, especially your knees.
If you can't purchase two pairs of shoes at one time, try buying a new pair of shoes halfway through the life of your current pair. Having a fresh pair of shoes as a reference will help you notice when your old ones are ready to be replaced.

http://running.about.com/od/shoesapparelandgear/f/twopairsshoes.htm