I've only shot weddings to help out a friend as an assistant informally, but I use my 16-35 so often I wouldn't consider going without it. If I had a 24-70, I'd probably take that, too, but covering the wide end with a zoom makes a huge difference.
It's good to see you're taking two cameras. I only have one, and it's absolute murder switching lenses all the time. I'd never, ever consider shooting a wedding as the primary photographer without a second body, both to cover multiple ranges and as a backup.