Fully charge all the batteries and take spare, charged batteries as well. Remember to have available at least one spare set of flash batteries - two sets if you are uncertain of the charge remaining in a current set. Don't attempt to photograph a wedding without having a spare camera. What would you do if you switched on your one camera and it just reported "Error"? Spare lenses are also good for peace of mind. No one carries two of everything, but lenses whose focal lengths are broadly equivalent or overlap are useful backups. In general terms, make sure you can cope if a lens gets damaged. A spare flashgun is a good idea although, with digital capture, it may be possible to manage without flash altogether.
Having two cameras available reduces the number of lens changes required. With two carefully chosen zooms, it may even be possible to eliminate the need to change lenses. Carry everything in a convenient shoulder bag or backpack which can be left open on the ground for quick access. A tripod can be left set up in the next location where it will be needed.