Two is better than one right? Honestly, it totally depends; the size of your wedding, what you want covered, and what is possible with the time and venue(s). I usually work alone but I'm always happy to add a second shooter onto any of my wedding photography packages.
When I'm not busy shooting my own weddings, I often second shoot for other wedding photographers, many second shooters are professionals in their own right. A second photographer can often cover things like groom prep when logistically it isn't possible for the main photographer to do so, they can also cover candids during the reception when the main is with the couple or taking group shots. I really enjoy working with other photographers when I second for them, it's always interesting to see how others approach the day and you can really take a lot away from it.
There is a train of thought that adding a second shooter to your documentary wedding photography package could disrupt the natural feel to the photographers approach of the day. Perhaps making you and your guests very aware of being photographed. Since taking on more work as a second I have had the opportunity to see it from a different perspective, and since I have decided that is not the case at all. For me, I blend in with you and your guests making everyone feel relaxed, and I always ask the same of my second, and only work with other photographers that have a similar style and approach to the wedding as I would. A huge benefit of adding a second shooter to your wedding package is that it can often free up the main to photograph more creatively, experimenting with different compositions and techniques - creating really unique images just for you.
That said, often one photographer can do the job. But this totally depends on the size of the wedding, and whether it's in one venue or across several. If I feel that your wedding will require a second photographer to cover your needs fully I will always advise you of this and I am always happy to add one to my photographic services.