One of the most frequent questions I get asked when shooting a wedding is ''Mike, why do you have two cameras?''... Apart from the fact it allows us to have a different lens on each body so we don't have to keep swapping them around, the main reason is for us to always have a backup.
At a recent wedding we were photographing one of the cameras died on us, these things happen but it could of been a nightmare, it wasn't an issue due to the fact we had a second camera on hand and a third in the car which we pack as standard for any wedding we photograph.
Gear fails when it's used a lot and it'll always be at the wrong time but we pride ourselves on making sure if this happens we have a backup and even a backup for the backup haha. We do the same with the precious photos we take, backing them up half way through the day and in 4 different places once we're finished.
There's many horror stories out there of wedding photographers not doing these things, we make sure we're not one of them....