A second photographer can be useful for several reasons, but isn't essential. It really depends on how big your wedding is, and how complicated the logistics are. When a wedding is large enough to have loads going on, and you want to ensure coverage in two locations, or from different angles of important moments, I can certainly book a second photographer, whose images will still be edited by me to ensure consistency across your album. It's also useful to have a competent second photographer in the unlikely event that anything happens to me on the way to your wedding (although this has never happened and hopefully never will).