I do not see why you would like to sell the D850???
If I only had one I would keep the D850 although I like the D500 better. The D850 is the more versatile tool. Great it is to have both together. Practical example: my standard reportage set is both cameras plus 4 lenses: 20, 35, 58, 85. Each of the lenses has two meanings for me, because I frame and not shoot for crops. Plus thev lenses backup the set. D500&58mm has the same "portrait meaning" as D850&85mm for me and while the D850&20mm is a true super wide angle, the D500&85mm is a longer outdoor portrait combo like a 135 style lens.
This set is very good for my style of shooting.