I see this question come up a lot on bridal forums and even photography forums. There are pros and cons to having a second photographer. What most photographers won't tell you that all comes down to the personal preference of the photographer and how they work best.
Portraits of the bride and groom in different locations were taken at the same time, before the ceremony.
I prefer having a second photographer (a professional not a student, assistant or camera holder) because it benefits me and my style of shooting. It keeps me relaxed and able to focus on moments and emotions around me. It allows me to take some creative risks, because I know another trusted professional is getting the safe shots. But I can, and often do, photograph a wedding alone.
I know other very talented photographers that prefer to shoot alone. They find a second photographer to be a distraction. They work better when they're in their own zone and don't have to direct another person. Sometimes they've had problems finding a competent second shooter that won't get in their way.
One moment, two perspectives.
My advice is to base your decision on the work of the photographer you're considering and not how many people they bring. If there is a second photographer included make sure you know the level of experience. Unless they're an employee, you probably can't find out who it is before you book. It's common for us to hire second photographers only a few months or less before the wedding. But you do want to know the level of experience the photographer hires. Are they someone who's using your wedding as a learning or portfolio building experience, potentially getting in the photographer's way? Or are they an experienced pro that your photographer has worked with and will help them perform their job better? Will there images actually be used? You do not want to pay for an additional photographer if none of their images are going to be used.
If a photographer offers you the option to add on a second shooter and you're unsure, ask for their opinion. Your timeline and the logistics of your day will have an impact on whether 1 or 2 photographers are better. Second photographers are helpful when you want getting ready coverage of both the bride and groom at two different locations. You or your fiance may not want to get dressed early enough to make it possible for one photographer to cover it.
Seconds are also helpful when there are restrictions on movements in churches. Often we're are restricted to the back, side aisles or balconies and not allowed to move. I've even been banned from entering the sanctuary! If a photographer mentions an assistant, clarify what that person does. An assistant typically doesn't shoot and is there to carry and guard gear, fetch stuff, help with lighting or organizing portraits.
The bouquet toss usually benefits from having a second photographer.