This works great in my setup: MBP 15-Inch, 2018, macOS Catalina 10.15.5, Installed via Homebrew Cask, LG 34" 5k2k 34WK95U-W via Thunderbolt 3, Sonnet Echo 11 Thunderbolt 3 Dock, Dell U3417W 34" via HDMI via Echo 11 Dock. Volume and brightness work on both displays. Contrast only works on the Dell (after enabling in the preferences) - no big deal. I can adjust brightness on both displays at the same time via the keyboard... that is spectacular. Volume controls work as expected - finally!! Fantastic!!