Do you mean you are setting up BE so BE is clustered, rather than just using BE to backup cluster nodes? If so, then there is guidance in the Administrator's Guide, but I wouldn't say it's as good as I would like if I were installing into a cluster. The Administrator's Guide seems to imply that CASO and SSO are beneficial in this scenario.
If you are just looking to back up cluster nodes, have a look at http://www.backupexecfaq.com/faq/getting-started/backing-up-veritas-cluster-server.html