What does the command
export DISPLAY=:0on terminal do ??
Is it possible to change the number from 0 to some other value..
And the EXPORT actually sets the variable to that value. It is used to tell X server to show what is comming next on that display. Example: How to start a a GUI software on a remote Linux PC via SSH
And yes, technically it should be possible to run KDE on tty6 and Unity on tty7. I have done it before (as in 2010) but I saw a question a while back about how to set that up and I failed to find a correct answer on how to do this with Unity. It needs to be done with the LightDM config and I can find the setting to change the port it uses (so change tty7 to tty6) but have not seen how to use both at the same time.