Oh. My. GAWD.
This post describes to a "T" my worst nightmare. My other half and I both shoot so we've always got it doubled up, but she does the formals and I back her up as a second shooter. However, in the beginning of the day, I follow the groom/groomsmen around while she goes with the bride/bridesmaids until we meet at the ceremony, which we both shoot from different vantage points with different lenses, etc. If either of us lost our data, we'd be missing half of the story of the whole day.
I also have the 6D, and I must say, Dual Card Slots are a MUST for jobs like that. I desperately need to upgrade for that exact reason.