Not sure about PNs as that's beyond my expertise, but from a technical point of view, yes you can have another server with CUCM on it for redundancy. You can even have more than 2 server, as long as you keep using the right OVAs and the overall capacity of the cluster does not exceed what we have defined for a 6K deployment.
If you go beyond 1K users and 1.2K endpoints, we will consider it a regular CUCM deployment.
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HTH
java
if this helps, please rate