This is a simple plugin based upon WarMarks Google Analytics Plugin allowing you to added a second Google Analytics code into your web pages. I redesigned this to add a second GA id that I needed to add to a clients website. I wanted to track and view the Analytics of the clients new site without going through the process of adding me to their account.
Once installed, you need to add the GA id's for both accounts (yours and theirs) and make sure you activate it. It will take care of inserting the latest Analytics tracking code into all of your sites pages.
You install this plugin the same was as you would any Joomla! Components, Module or Plugin ==> Extensions ==> Install/Uninstall ==> Browse to where the package has been saved ==> Upload.
Navigate to Extensions ==> Plugins and sort or find "System - Google Analytics Plugin by 465-Media.com". Select "yes" under enabled, add the two Google UA id's (with the UA in front) in the parameters and hit save or apply.

The is a Free Plugin, but you must register with this site. You can do so here. Once you are registered you can login to your account and it will be listed in both "Your Subscruptions" and "Free Subscriptions" for you tod download.
Support can be found here.