Updated: Mar 2
When a lot of photographers are first starting, one of the first things they ask is "What kind of wedding gear do I need?"
While it's true that you don't need 2x $3000 camera bodies, there are some fundamental things to look for in a camera body. Dual card slots being one of them.
The main, and honestly most valid reason, is security. We're hired to do produce under pressure. To get the shots and be unquestionably reliable. When shooting an event that is unrepeatable (hopefully), having a memory card give out on you and lose some or worst case all of the memories is literally the worst case scenario.
The argument that "I've never had a card fail" or "a camera body failing is more likely" doesn't change the fact that instant backup is invaluable. Yes, cameras can fail, lenses can break and gear can get stolen. We still need to do everything that we can in order to safeguard and guarantee that we deliver.
This is why not only do I shoot raw to both cards, I back the images up on the spot after an event for a third layer of security. Like the saying goes, two is one and one is none.