First of all, make sure your main photographer is a good one and knows what to do with all lighting conditions. If your main photographer is lacking craft skills, having 2 or 3 photographers will not leave you awesome pictures.
Now assuming that you hired a really good photographer, having a second photographer will benefit you:
* if your wedding has many different locations * you have 200 or more guests and you value candid pictures most * if you have a limited time for bride & groom, bridal party and family picture sessions * in many cases if you have a budget for it
For small and average size weddings taking place at one location or one location for the wedding with 2 getting ready, you may just need 1 great photographer. I often go from one getting ready location to another if I have enough time in the schedule. Btw, I work on schedule with every couple months before the wedding day to make sure everything important is addressed.
Many couples find a good photographer then choose another one because they received similar rate for 2 photographers, which seams like a much better deal. Some studios or independent photographers offer attractive coverage with 2 photographers. They look great on paper, but in reality the second photographer in a very inexperienced one. So if you make a budget decision and hire an average photographer with a second one who should be called assistant in training, you are still overpaying.
The main photographer takes about 70% of the wedding pictures. Make sure you hire a photographer, ready for anything in any lighting condition, someone who has a creative eye. He/she will always be a better deal than 2 or 3 average photographers.