I am terrible in how I treat my cards. I have four or five cards that have gone through the wash (and dryer) at least one time, in one case three times.
This is true for both SD and Compact Flash cards. None have failed once I got them up and going.
I did have a Lexar card that was dead on arrival, and the company sent me a new one (after a month). I had a Sony card also dead on arrival, but they declined to honor their guarantee. Never got back to me after calls, emails and letter. Makes me wonder how their "pro" service that they launched with the A9 is going to do. Perhaps it'll lead to a better model for them, but as of now I'm skeptical of them.
I buy Komputer Bay compact flash cards now because they're generally as fast as the Lexar/SanDisk ones, significantly cheaper, and apparently as reliable. For SD cards, I buy very large size cards that happen to be on promotion at the time.
Most of my cameras have two slots, but I use one as overflow, rather than backup. That the 6D2 appears to have just one slot doesn't disappoint me but so much. I would have used the second slot for overflow in situations such as timelapse setups, where memory requirements are huge.
The requirement for a 2nd slot strikes me as one of those "pro hangups," like not wishing to have a vari-angle screen or not wanting a fast or wide lens to have IS.
The real risk of card corruption is centered on how the camera writes to the card, rather than a hardware issue with the card. As such, a wedding pro should be swapping cards out with some frequency, protecting the images from the camera. If a pro swaps a card out every hour, then a catastrophic failure might wipe 20 percent of the images. If a pro leaves two cards in the camera for an entire event, I'm not sure they're more protected.