It seems discussions about dual slots are always somewhat controversial but I found this article that has some great points.
A few personal comments for single card cameras: I don't exactly agree with tethering though as I find it terribly unreliable or inconvenient. A separate backup device is highly recommended, but I wouldn't use a WD Passport Pro (or Lacie Copilot) as both are exceptionally slow and not really suited for pro work. There are much better solutions offered by Gnarbox and NextoDI (which I use) which also has CFAST and UHS-II support and are SSD based. They can also do a dual copy and verify at the same time.
Is it Irresponsible to Shoot a Wedding on a Single Memory Card?
A year or so ago, I wrote an article on why I switched from Nikon to Sony. A few people asked why I didn’t move to the Nikon mirrorless system. And the
petapixel.com
A few personal comments for single card cameras: I don't exactly agree with tethering though as I find it terribly unreliable or inconvenient. A separate backup device is highly recommended, but I wouldn't use a WD Passport Pro (or Lacie Copilot) as both are exceptionally slow and not really suited for pro work. There are much better solutions offered by Gnarbox and NextoDI (which I use) which also has CFAST and UHS-II support and are SSD based. They can also do a dual copy and verify at the same time.