Guess how many SD cards that leaves the data on?
One. Still.
Again, there's a reason dual SDs are standard fare on any pro-sumer or higher level camera. SDs fail. SDs get lost. SDs get corrupted. SDs get accidentally deleted. Data in two places is a minimum requirement.
In the case of Nikon cameras, the default setting for the second slot is overflow, not mirroring, although it can be used for that. Personally I use one card for JPEG and the other for RAW, but then I’ve never lost data on an SD card.