At the launch of Acrobat DC existing installations of Acrobat XI have been removed without notice. This raised a lot of protest in the Internet.
It took quite long until Adobe offered a solution. Since the October update there is now a setting in the installer which allows to choose if existing Acrobat installations are removed or not. By default they are still removed!
For a lot of users this new setting comes too late. For them Adobe has published instructions for the re-installation of Acrobat XI. First Acrobat DC must be uninstalled. Afterwards Acrobat XI is installed. The instructions don’t mention that one has to re-install Acrobat DC afterwards (of course without the option to remove older versions!) in order to use both versions on the same computer.