The Ubuntu and KDE developers are working together at making Unity 8 and KDE coexist, permitting the users to have both the two desktop environments on the same system.
Unity 8 is still under massive development, being used only on the Ubuntu Phone so far, but it will be adopted on the desktop version of Ubuntu as soon as it gets ready. Most likely, this will happen when the convergence will be reached and both the desktop and mobile system will have Unity 8 and Mir as display server and will be using snap packages by default.
The KDE developers have to package the desktop environment for each of the systems that is using it by default, Ubuntu included.
According to Michael Hall, one of the Canonical developers, Unity 8 and KDE could be made to coexist, as the Plasma Mobile system is built on the Ubuntu SDK but uses KDE as default. Some modifications to the login manager still need to be done, but nothing is impossible.