Yes. If both photographers know each other well and therefore work well alongside each other. Having two photographers who don't know each other and are of a differerent skill level may spell disaster for some of your photos.
Usually I'll get my second shooter to capture the family's reaction during the ceremony, allowing me to give 100% of my attention to the couple. A second shooter can also capture a variety of different and unusal perspectives during your day, whilst the primary photographer will likely be making sure to capture the safe shots, before moving on to different angles etc.
Another time when a second shooter can be invaluable is before the ceremony commences. I usually hang out with the groomsmen whilst my second shooter stays with the bride, capturing those sometimes tense moments as she prepares to walk down the aisle. Meanwhile, I can capture the groom's reaction as the bride walks down the aisle as well as that moment when the bride walks towards her groom.
Having said all of that, I have photographed heaps of wedding el solo and the images I deliver to couples have always been amazing. A second shooter just adds a bit more variation and perspective. If you check out my photos, the two black and white photos of dads, one laughing and one crying, are both taken by my amazing second shooter/wife Corinne.
So in sum, no need to panic if you realise you have only hired one wedding photographer. However, two professionals who work well together are generally recommended to capture your special day in all its glory.