The Cache Array Routing Protocol (CARP) can be used to create a single logical web cache that is distributed across multiple TMG firewalls in a TMG firewall array.
Check out this article to see how you can use CARP to speed up web browsing in your environment
http://www.isaserver.org/articles-tutorials/configuration-general/Content-Caching-Forefront-Threat-Management-Gateway-TMG-2010.html
HTH,
Deb