CF Cards were relatively physically robust, SD cards less so. Many pros and amateurs like having two card slots. Writing RAW to one and Jpg to another for backup. A few recent cameras have had a lot of flack for not having two card slots but card technology has improved a lot recently.
While CF and SD cards are prone to failure the new designed XQD and CFexpress have had no recorded failures due to hardware. Now all cards have a recommended number of read writes, and it is when you exceed these you start to get errors. Errors with these new cards are due to exceeding these recommendations.
If your camera uses one of these new types of card I would be less worried about needing two slots for safety. Some photographers who after a shoot hand a card to a third party but also need a copy for themselves then yes there are ways cases for two slots, sometimes event photographers will print out a quick jpg and sell to the client on the day so having jpegs to one card and raw to the other is handy from a work flow perspective, but for many of us one card slot is now enough.
No matter how many card slots you have I would recommend you replace your cards regularly. It is likely now age and exceeding the number writes that will get you rather a the card failing early.
