What lenses are you shooting if I may ask? If you have the "kit" lens with it already, the 24-105mm F/4, I wouldn't even think about getting another body yet. I'd pick up two solid primes, or a 24-70mm, and a 70-200mm before that happens. I agree with the majority this time, save your money until you can justify big purchases. It's really rare that a camera becomes unusable during a wedding, unless you're careless, accident prone, AND like to hang out right by bodies of water like pools and fountains, or even a very tall staircase. I have a couple of 6Ds and both are past warranty already, peeling, fading, scratching, etc. One of them even has over 200k actuations. Both are still shooting like the day they were bought. And like you said, the main shooter has a backup already. The chance of both of you needing to use that backup as a main camera at one event are so slim that there's actually a much better chance that the bride leaves the groom at the altar.