If you are running multiple sites that share a common root domain, it is easy to setup a system where logging into one logs you into all of them.
Designate one of the sites to be the master site.
For the other sites, go into
directory, and remove the
directories. Now create shell-links to the directories of the same name in the master site.
Then, in all the sites, go into
and edit the
file. Add the following line:
Substitute the mydomain.example for your own domain.
Clear your browser cookies after making this change.