By making some modifications to the GA code it is possible to send data to two Analytics accounts from one web page.

For urchin.js:

<script src="http://www.google-analytics.com/urchin.js" type="text/javascript">
</script>
<script type="text/javascript">
_uacct = "UA-XXXXXXX-1";
urchinTracker();
_uff = 0; // Reset flag to allow for second account
_uacct = "UA-XXXXXXX-2";
</script>

For ga.js:

<script type="text/javascript">
var gaJsHost = (("https:" == document.location.protocol) ? "https://ssl." : "http://www.");
document.write(unescape("%3Cscript src=’" + gaJsHost + "google-analytics.com/ga.js’ type=’text/javascript’%3E%3C/script%3E"));
</script>
<script type="text/javascript">
var firstTracker = _gat._getTracker("UA-XXXXXXX-1");
firstTracker._initData();
firstTracker._trackPageview();
var secondTracker = _gat._getTracker("UA-XXXXXXX-2");
secondTracker._setDomainName("none");
secondTracker._setAllowLinker(true);
secondTracker._initData();
secondTracker._trackPageview();
</script>

Please note: this applies to XHTML Transitional doctype written in utf-8 encoding. You may also download this post as a plain text file for easy copy & pasting.

Enjoy!