You would configure 72 vm ports for primary, and another 72 ports for secondary server on the callmanager. I think you got this part.
On the secondary unity server, you would only get two ports initially because you are supposed to configure the server as a demo-licensed unity server.
After you run the failover configuration wizard, the license from the primary is replicated to the secondary and 72 ports will be available for configuration for secondary unity server.
This doc outlines the steps:
Configuring Failover on the Primary and Secondary Servers
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_feature_guide_chapter09186a00801b924e.html#wp1280717
You probably have these links already but I am putting them for others.
Cisco CallManager 4.0 Integration Guide for Cisco Unity 4.0
For a Cisco Unity Secondary Server
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/prod_configuration_guide09186a00801e9e37.html#wp1215272
White Paper: Cisco CallManager Port Configuration for Cisco Unity Failover (Cisco Unity Versions 4.0 and 3.1(2) and Later)
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_white_paper09186a0080114679.shtml#1022055