You can have any version installed and even run at the same time as you want.
I strongly recommend to stay in the very same version. Don't switch via IDML between CS5.5 and CS6 (or any other version).
If the license allows it (as most Adobe licenses do) you can install it twice but only use once at the very same time. If you need to do work from home, ask your boss, if he allows you to install the second version on your home computer.
