I remember a day when you didn't have dual rolls of film in the camera and one wrong bump of a lever and the whole roll would be gone. 2 Bodies is a must, 2 shooters with 2 bodies is much better, but I don't think 2 card slots are an absolute requirement. Though yes shooting the same moment to 4 instead of two cards does take extreme remote possibilities and make them even rarer. I haven't seen a card corrupt and fail in over a decade, though I have seen an SD card get smashed beyond being able to get inserted into a reader, when it wasn't given proper care (CF card in the same situation survived fine though). It's been my experience that you're far more likely to see an Err message or have a battery forget to be charged or finish it's life span or see a camera and lens get dropped and break before seeing a card fail or corrupt. Even then when cards get corrupted, you can typically recover most to all of the data on the card with some patience and software than SanDisk and Lexar tend to ship out with their cards.
The 6D would be an okay option, if you could pair with with your 60D and have 2 bodies going at the same time. However I would caution you from the f4, I understand your reasoning of stops of ISO trading stops of glass, however there is more to an image than noise levels and the f4 DOF particularly with wider focal lengths would make a lot of backgrounds much harder to work with. You have wide covered so you can do group shots with a lens that maxes out at f4 (and probably stop it down some to make sure everyone is in focus), with the gap from 40-70 I personally would go for a fast 50, being required to zoom with my feet, but me much more capable of handling less than ideal backgrounds, and even more capable inside dimly lit churches/temples/etc. that have no flash policies. 24-105 is a good lens, but if you're planning on eventually getting the 24-70, why go through the hassle of replacing it when you could put similar to less expense into something to supplement? I understand you're looking to take advantage of deals in the next week, otherwise I'd recommend0 trying out the new glass before deciding to upgrade (though the 50s do seem to do much better after a full frame transition).