Hi Sam Lewis,
and a belated welcome to the forum!
Allow me to offer a slightly dissenting answer to my esteemed co-posters.
I believe that it is necessary and also very useful/helpful to have a second camera body as a wedding shooter.
There is the obvious concern that your primary somehow fails you. There are no second chances and if you can't cover the whole wedding because your one camera stops working, not only are you hooped, but you will have a very sad and frustrated couple wishing they had hired someone else. And if you actually care about your clients, you cannot allow such a situation to occur. This is their one day - in some cases THE most important day in their young life....well you can imagine. Of course, this is about riding the odds here. How often does a camera break - not often.
But beyond regular equipment failure, there is drunken uncle Bob, grabbing your camera cuz he wants to see it and dropping it in the punch-bowl..lol.
Secondly - and this to me is the real reason why having to is better..much better: You can be ready to shoot with two different lenses at once.
Indoor weddings sometimes, even often, takes place in poor light. Especially the party after. For that and creamy bokeh, one camera may be mounted with a very fast lens - say an affordable 35 or 50mm F1.8 prime. The other may be mounted with lens with more reach.
Or, whether it's well lit or not, one may be mounted with a wide (say Tokina 11-16 F2.8 constant aperture) for the group shots and the other with a lens with more reach (say 85mm F1.8 AF-D) for example.
As a professional wedding photographer - eventually commanding $1.500 and up for a wedding - I don't think there is any excuse for not getting the images because the camera stopped working.
But that is just me..