^^ because I want to try KDE and am using unity launcher for lots of apps and don't want to make loads of short-cuts.
I have had no problems with running them along side each other. You will select which environment you wish to log in to on the login screen at start up by clicking that little white circle. Assuming you have no other environments installed the list should just consist of "Ubuntu" and "Ubuntu 2d". When you install KDE you will see the two "Ubuntu" environments and the KDE Plasma Workspace. Click on the KDE option and input you password and you should be ready to go.
One side effect of doing this is that you will have programs from both environments listed. For example, you might have gedit as well as kde's text editor list under programs. You might also have KDE's settings manager and unity's settings manager both listed, which can get confusing. Other than that I have had no problems running KDE,mate,gnome,xfce, and unity all on the same machine.
In fact right now I am using a new install of xubuntu with Unity installed with it! Everything works just fine.
I would highly suggest reading this: http://psychocats.net/ubuntu/kde