BigFix licenses are based on the endpoints, not the number of BigFix Server instances. Your license entitlement does allow you to have 2 BigFix Server instances (i.e. 2 mastheads) and to allocate the number of endpoints as desired. However, Clients can only report to one BigFix Server at a time, and while we provide a Fixlet to have Clients switch their masthead, you’d also have to migrate BigFix users/operators, settings, policies, etc…
Perhaps you could give us a little bit more insight on what it is you are trying to achieve? Is your existing masthead tied to a particular server name or IP (rather than a DNS alias)?