My general rule of thumb is that I can best represent a wedding of 150 or under by myself, but beyond that I always recommend a second shooter. The reason for this is really because with a higher number of guests, I don't want to take my time away from the bride & groom because I am worried about capturing every single person who attended. If the photographer you are considering is like me, most photographers have a great network of shooters that they contract out for this... mine have actually become close friends! We have a similar approach and and shooting style so our work blends together nicely. They shoot, provide me with the files and I edit the whole wedding together to keep things as consistent as possible.
"How do I use a second photographer"? I usually start off by figuring out how long they are needed. I like to avoid having a second photographer with me all day because I don't want people to feel like there is a constantly a camera in their face. It typically ends up being that the second photographer shoots the groom and his guys getting ready as well as reaction or balcony shots of the ceremony and the recessional. Then we move on to the family/bridal party photos. Depending on the timing of the day, they either come with me to capture candids OR head to the reception to take details shots. They also capture whatever part of cocktail hour I might be missing. Sometimes they stay longer if table shots are requested, but after cocktail hour I'm generally ok on my own.
"I want a second photographer to capture my groom's reaction when he sees me as a bride for the first time!". The is my favorite moment to capture at a wedding! Truth is, a second photographer won't really be the one to capture this. The photographer you hire will position his or herself somewhere in the isle to photograph the processional. Once the doors open they should snag a few photos of you first and then turn around to capture the groom. Then its back to the camera being on you. All of this happens fast, but if you are a skilled and experienced photographer it should not be a problem. This is a beautiful moment to preserve, but DON'T hire a second photographer for this moment alone. In almost ten years of photographing weddings, I've never missed this shot.
Some last food for thought... I never take a second photographer when I photograph the bride and groom. I like this to be as intimate as possible so the couple feels comfortable and relaxed. I believe the best photos are taken when this happens, but also that it is important to take a break from the craziness and enjoy being together. In most cases it will also be the first time they get to really be alone after the ceremony. This time is short and sacred. Also, it is usually the only one on one time I get with a couple on their wedding day so I naturally prefer it to best represent me.
To wrap this all up, I know a lot of people go with multiple photographers for security, which is totally understandable. If you feel this way, just understand the vibe that multiple photographers will bring to your day and make your decision accordingly. Don't forget to factor in the type of wedding you are having, the amount of guests and your personal style. Whatever you do, hire a photographer that you feel confident with and trust them implicitly. They will feed off your trust and go above and beyond for you!