I cover weddings with two 1DX MkII's. Principally, as Don and I have already said, to negate the need for lens changes. There are not many pro wedding shooters here, so I stand by my comment which had a very narrow definition.You wouldn't cover a wedding or an assignment with one camera body? You couldn't use that excuse that these bodies hardly fail if something were to happen during their "hopefully" once in a lifetime moment. Having the same kind of backup body is helpful and most preferred in many ways but the 7d series makes a very nice backup body to the 1dx series , shaving some weight off the kit.
I have covered hundreds of assignments with one camera body, including my own wedding.