I assume you're ignoring my posts #312 and #319 as they don't tell you what you think the answer should be. Nikon haven't implemented your wish. The D850 and D500 have 2 different speed cards and the D5 with 2 of the fastest card slot currently available gets a slower write rate in backup mode than in single slot mode- in your approach the D5 is gimped because you have to choose between fastest throughput for images or backup mode.
Even the 1DX ( I cant find info on a 1DX2) with 2 CF slots suffers the same issue, link and extract below.
"The EOS-1D X is the first Canon camera to have dual CF card slots. Performance using both slots was tested with two Lexar 1066x cards in the camera. In 30 seconds of continuous shooting, recording the same images to both cards, RAW+JPEG reached 63, RAW 93 and JPEG 198. This is slightly less than the 84, 126 and 259 shots observed when recording only to one card. Setting the camera to record separately (RAW to one card and JPEG to the other) yielded 87 shots versus 84 when recording both RAW+JPEG to one card." See link below
https://www.cameramemoryspeed.com/canon-1d-x/fastest-cf-cards/
When I grew up there was a saying that a bad workman blames his tools and a good workman knows which is the right tool for the job. You might be a good photographer, I don;t know as I've not looked at your website, but as a professional you aren't very inspiring with the attitude shown. When your clients point out issues with things you would like to do, do you listen to them and take their advice, opinion and knowledge on board and recognise that they might know more than you or have described that what you want doesn't actually exist anywhere in the world from any manufacturer? If so how long do they typically stay as clients?