You can configure the Agent Handler Assignment list under Menu > Configuration > Agent Handlers. Here you can assign the Handler Handler Assignment Rules, which configure the Agents to use a list of Agent Handlers in order. This 'Policy' is used to build the SiteList.xml for the given system agents.
Thanks very much for the prompt response. Appreciate it.
FYI as soon as I added secondary AHs to each of the Handler Assignment rules, my clients got an updated sitelist.xml with the secondary AH upon their next policy refresh.