Q: Will my shoes last longer if I alternate two pairs?
A: Not really. Running shoes aren't like your body; they don't need rest days. Shoes are made of foam and rubber, and daily use doesn't significantly affect their durability. Two exceptions: if you run twice a day, or if you get your shoes wet frequently, then alternating them might be a good idea
to let each pair dry out better. http://www.runnersworld.com/article/0,7120,s6-240-319--268-0,00.html