A couple additional thoughts based on the comments thus far...
1. Two cameras are better than one since you have a backup: While I do tend to take an extra body with me on trips, I also want to point out that in over 30 years of photography - many of those years shooting professionally and putting massive numbers of images through the camera - I've had exactly zero camera failures. I think for the casual shooter, a single body is just fine, especially if it's a pro caliber body. Sure, you take a risk, but it's a small one IMO with the benefits of the D850 outweighing the backup benefits of a D500 + D810. IMO.
2. One lens per body, no switching lenses: OK, this isn't a bad way to go and I have done it before. Especially if one is set for landscapes and one for wildlife.
3. The D850's is slower than the D500: From an AF position, I disagree. I've shot nearly 30,000 photos already with my D850 (and the same number or more with my D500) and I find the AF equally fast for action in both bodies. The D500 is 1 FPS faster than a gripped D850, 3 FPS faster for a non-gripped D850. The D500 also has a better buffer, although hitting the D850 buffer is a rarity in real world shooting if you're set up properly (i.e. using 12 bit after ISO 400, taking advantage of crop modes, etc). IMO, having a the full frame body with what amounts to a DX camera built right in outweighs the minor advantages the D500 has over the D850 (gripped D850 - non gripped, the D500 starts looking better).
The reason I seem so hell-bent on the D850 is simple. After using it the last several months, I notice I miss a lot less action shots that I do with other bodies thanks to the pixel density. In the past, I'd shoot my D5 and D500 for wildlife and I was always swapping bodies for more or less reach, often missing shots in the process. With the D850, if the subject is only taking up the DX area and I need the shot, I go ahead and fire away, knowing I'm capturing what I would have had with my D500. As the subject gets closer, I don't try to switch to my D5 - I just keep shooting and putting more pixels on the subject as it fills more and more of the frame.
One final note - keep in mind that I am primarily a prime shooter so the advantages of not swapping bodies means more to me than a zoom lens shooter. However, I did want to share my experience.
More D850 info / setting, etc on my review page:
https://backcountrygallery.com/nikon-d850-review/Finally, a few D850 photos just for fun (mix of action and stills)
A couple additional thoughts based on the comments... (