May I ask why two D850s, if weight is an issue. Why not swap one out for a Z7 II.
The Z7 II is BRILLIANT perfectly fine as a everything you want to do camera and it doesn't do anything really that the D850 cant do, other that sensor stabilisation which is becoming more important as we age. Yes if your super articulate and in critical situations and you are a shooter that relies on tracking (and I ask how many times are you in that situation) the Z7 II is absolutely stunning with the Z glass, don't bother with adaptors the befit is in the Z system is the Z glass, light weight, small, colour, clarity, the Z glass its about I don't know maybe 5% to 10% better, but the combination in colour accuracy is amazing.
We have couples in the club that travel full time doing photography all around the world, they have the Z7 II now and they say the Z series is so good for everything especially travel, they also do birds in flight and wild live, macro, street the whole lot, and love it, many other members say they don't have an issue with action either, I mean they don't use in camera tracking, in fact I will be honest, I have never once used tracking in any camera period, I have no complaints. I see for many its nice and become a critical issue for them if it doesn't work well, but I am old school shooting, for me it delivers results.
I have seen so many people in our club address this issue of travel and weight, many went to Fuji, but came back to the DSLR then the Z series came out they went to the Z6 and Z7 and they haven't locked back, very few of them use the tracking feature. Many keep the D850 as well.
Its the journey that matters, not just the destination, try a Z7 II on hire first with a good 70-200 Z lens and I think you will be sold instantly.
Only an opinion OZ down Under