It looks like I have a stable configuration of the Dell 8K display (UP3218K) and a secondary 4K display running on RHEL 7. The default configuration with Nvidia driver 384.90 appears to be to create two tiles, each 3840x4320, and place them on the same physical 8K display. If you set one tile as the full resolution 7680x4320 screen, you get a laggy mouse, among other problems. I don’t know why this works with tiles, but it works. Whether it is persistent and consistent, I don’t know. I’m documenting this for others’ use in getting the 8K display working.