There are lots of reasons to have two photographers at your wedding.  The most important is final product quality with nearly simultaneous but different perspectives of events during your wedding day, thus enriching the story telling of your pictures.  In the first two pictures below, the bride was posing for a formal picture at the same time the guys were having a group picture. 

The second example are pictures taken at the same time and at the same location, thus providing two nearly simultaneous perspectives of the same event.  This also greatly enhances the story telling of the pictures.  Your wedding day is a once in a life time experience and the memory of that day are best preserved through photographs.  Skimping on the photographer budget really raises your risk of not having those great memories in the future.  Below are three examples from a recent wedding of different perspectives takening at the same time and place during a wedding.   

Advantages of two photographers:

1.  A very rich set of pictures of the most important day in your life.  This richness will enhance your memories many years later and helps create wonderful albums.

2.  Reduces the chances that something important will be missed.