The decision to do one or two photographers is usually a big one when it comes to your budget. Let me say, there is no wrong way, one photographer or two, but having two photographers does offer you more.
A great thing is it allows a photographer to be in two places at one time. Sometimes couples get ready in different locations and both want those getting ready moments. It allows for more time for both wedding parties and to get all those photos you're wanting. Same thing with the reception. While the main photographer is busy getting those family portraits, the second can be at the reception grabbing guest shots and detail photos of centerpieces, and don't forget the cake!
Having a second shooter allows for more coverage during the ceremony, more creative angels during your couples portraits and let's not forget.... you get A LOT more pictures in your wedding day gallery.
Another great thing is that although the two photographers may work closely together and understand one another's style, you still get photos from a unique and different perspective. One photographer might display your rings here, but the other may grab them and set them up in a completely different imaginative way. Thus giving you two very different looks in the same setting.