I do have two XQD cards in my D5, set to backing-up. My newer D850, which I have yet to use for anything serious, has one card inside it, at the moment.
I am not a “pro” photographer, but before my recent retirement, did shoot images, for official purposes, documenting crime scenes, crime victims’ injuries, and evidence. When it seemed prudent, I would shoot two sets of images, with two 7D cameras that had single card slots. When I upgraded to 7D II cameras with dual card slots, I was less likely to shoot the same images with two cameras, but sometimes, still did so, anyway.
My wife shot death scene images for the Medical Examiner’s office, for 21 years. The film SLRs, of course, held one roll of film. When she was issued a Canon 40D digital camera, it had one card slot. When she was issued a D300s, with two card slots, she used the second slot for overflow. When she was allowed to use personal cameras, she used the second slot of her D7000 for overflow. Now retired, she uses the second slots of her D500 and D850 cameras for overflow. (She wants a Z7, with the 24-70/4 Z lens, like the one we handled at a Nikon launch event in Houston last evening. )
A memory card is a point of failure. XQD cards are sturdier than SD cards, and according to some sources, are better-built internally, too.
A card slot is a point of failure. It is CF card slots that have the delicate pins. XQD card slots have no such pins.
Cameras have multiple failure points. The images may never make it to the card(s).
I, the shooter, am a point of failure. I am the weakest link. If I have one card, I had better not drop or damage that card, or negligently delete important images.
I think it is likely that there will be more-pro-like Z cameras, in the future, and, inevitably, a more consumer-oriented Z camera. I believe Nikon felt compelled to announce something before Photokina, that was more substantial than a teaser. The Z7 is a probe, launched into the high-end enthusiast market. The Z6 is a probe, launched into the mid-level enthusiast market. These probes send information back to the Nikon home world, which is valuable for subsequent missions.
-- hide signature --
I wore a police badge and pistol, and made evidentiary images at night, incorporating elements of portrait, macro, still life, landscape, architecture, and PJ. (Retired January 2018.) I enjoy using Canon and Nikon gear.