Hi Cas,
you don't need an extra one. You can point directly to the cluster.
On the Unity box you'll still need the Exchange System Manager, but you can have Unity point to a virtual server on a cluster during Configuration Setup part 2.
if you're using Active/Active, point Unity to the virtual server that is running the MTA (there is only one instance of this service for Active/Active clustering).
Here's a URL on the config:
http://www.cisco.com/warp/customer/788/AVVID/mailstore-no-start.html
regards,
kurt