I'll assume that you have one callmanager cluster configured already in Unity. If you did, then decide on how many ports you want to dedicate to each callmanager cluster. If one cluster has 75% off all the calls, it would make sense to spilt the unity vm ports to a ratio of 75/25.
In \commserver\Utilities\Utim\UTIM.exe, if one cluster already exists with all the ports, then highlight the ports on the ports-tab page, and click on REMOVE PORTS.
Then right-click on the callmanager integration, and select New Cluster. You would then enter the new ccm-cluster's ip address. For the ports, you can have the same name as the first ccm-cluster but I think for you own sanity, you might want to give it a different name (the name has to match the vm ports on the 2nd ccm cluster).
I think to answer your question, if the calls are equal for both ccm clusters, the create one cluster with 50% of the ports, and then create a second cluster with tht other 50%.
The part I am not clear about is what you mean by two sets of ports.
You have another option to use Intercluster Trunk to deliver the voicemail to the other CallManager. This way you do not limit the number of UNity ports for the CallManager, and you simply share it as needed. You would need to however still create the integration with at least 1 port for MWI.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.