new to Kubuntu and relatively new to Linux overall. I installed Kubuntu 16.04 on Desktop with add-on Radeon HD 5850 GPU. I'd previously used this exact hardware to run dual monitor. I plug a DVI (my main monitor) in and an HDMI that runs to a HD TV in another room. When I installed 16.04 the setup worked perfectly. All I had to do was switch Audio Configuration to change sound from one to the other. Yesterday I updated the system and suddenly, when the desktop is on the TV is black. So I tried to change the configuration with the GUI Display Configuration Utility but could not fix it. If I leave the HDMI plugged in the Display configuration shows that both are recognized, but only one screen can be seen at a time. Unplugging HDMI brings the regular monitor back, plugging the HDMI in switches the output to the TV. When I login, I can see the dual screen on both until I finish the boot, then it starts this behavior.
I have a similar setup, different gpu version and using VGA and DVI. I am facing the same problem from the most recent update. I previously had both monitors working with the 18.104.22.168 kernel.
During startup see if there is a new kernel installed and try booting into an older one.
edit: also try this Dell 3543 Kubuntu 16.04 can't get output to 2nd monitor on HDMI i haven't had a chance to give this solution a go