Having a second photographer isn't necessary but available if you would prefer to have one. I would suggest having a second photographer in these cases:
If the groom is getting ready in a different location than the brideIf you want cocktail hour coverageIf you have access to balconies or high locations for your ceremonyIf your wedding has 200 guests or largerIf you are not doing a first look and only have one hour to document family formals and reception details.However, having a second photographer does not mean you will have lower quality coverage, so don't feel pressured to add one to your package.