We have a Unity 4.0(5) voicemail only configuration. The message store is off-box.
In the integrations, we see that there are 2 clusters configured, however these clusters point to the same callmanager.
We have Cluster 1 with CallManager1 as primary and CallManager2 as secondary and a Cluster 2 with CallManager2 as primary and CallManager1 as secondary. So actually, they configured two clusters in Unity but there's only one cluster...
If the MWI dials out, it dials out on both last MWI ports of the integrations.
This Unity was not installed by us and we wonder if there's a reason for configuring it like this and if it could pose any problems.
The configuration you have inherited is sometimes done for load balancing. See here for example:
Unfortunately, the Unity Integration/Cluster design was not really intended for this. The MWI behavior you see is an expected downside of the configuration. By reconfiguring your ports such that all are in a single group ("cluster") you will gain some efficiency in MWI but you will loose some load balancing on CUCM.