I am also having an issue with monitors, but I am running 3 total (one 4k display on my laptop, and 2 2560x1600 screens attached via a USBC docking station). I am running a Dell XPS 9560 laptop, and with the live install USB, I get two displays working, and the "taskbar" (sorry, I'm a Windows guy) doesn't display on the second screen. When I complete the install and I rebooted, I updated all the installed packages, but now I get no "dock" or "taskbar" on any screen.
Additionally arranging the screens left to right as they are on my desk 4K, 2K, 2K side to side aligned on the top of the screen, I can't get all the screens to even turn on (if I turn one of the 2k resolutions, to half res, it allows them all to turn on). If I stack the two 2k monitors to the right of the 4k screen, I can have all 3 on at full resolution.
Is there something I need to do to fix that? What do I need to do to get my "taskbar" or "dock" back with all 3 monitors on? I know this laptop has optimus, and I'd be fine with the lower battery life of turning it off if need be. I tried to use some steps I found for Fedora, but those did not work, and I ended up having to delete my installation and reinstall.