From Google on multiple tracking codes:
Installing multiple instances of the Google Analytics Tracking code on a single web page is not a supported implementation. We suggest you remove all but one instance, and make sure you have the code from the correct view installed on every page you would like to track.
You may be interested in the answer to the question How to display combined stats for multiple domains in Google Analytics? by dan
According to this post in the Google Analytics forum, the new Universal Analytics (in Beta) provides the ability to use the same tracking code across all your sites.
This will require creating a new profile, which should then display the option to use "Universal Analytics" instead of "Classic Analytics". For more on how to setup Universal Analytics, see this.
For tracking multiple domains with Classic Analytics, see this: Google Analytics - Set up cross-domain tracking for multiple domains.
There are also numerous web analytics applications that can be added to analyze your web server's logs for all your sites. You might find it easier to use one of these apart from Google Analytics to monitor your web server's usage. For a list of these, see: List of web analytics software