Yes. Back when I was speaking, and working with the NSA (National Speakers' Association, not the security service), it was common for speakers to have backups who would be able to pinch hit for them in the case of sickness or disaster. (Sometimes, it was as simple as all the flights being grounded due to a major weather event, and two speakers swapping assignments because they were within driving distance.) It's not as common in professions that don't require you to physically relocate to your client's location every time, but I know of plenty of authors who have had other authors fill in for them at conferences or other events due to various crises. And I had a friend whose main job was ghostwriting books for another, more famous author.