I believe what you really want is multiple Calendar servers, while maintaining a "unified" view of all the Calendars distributed across the servers.
This is possible using the Database Wire Protocol (DWP) service of the Calendar server. Go to docs.sun.com and search the JES books for DWP to get a list of docs to read. The basic steps are as follows:
1. All Calendar users should/must use the same LDAP base (not sure about this)
2. Modify ics.conf to enable dwp protocol and specify which servers are using DWP
3. Modify users LDAP entries to include the icsCalendarDWPHost object and set icsDWPHost to the Calendar server where the users Calendar will be stored.
I've only just started testing this, but it seems to work ok.