Here's the set up using iCal Server on Mac OS X Server 10.5.7:
User A (boss) has an account on the iCal server. User B (admin asst) sets up an account in iCal with User A's account name and password and gets full access to User A's calendars.
In some brief tests of creating and deleting calendars, it looks as though this will work fine (simultaneous connection via the same account to the same calendar by multiple users) and accomplishes two goals:
--It allows either User A or User B to mark an event or to do as private. (When User B is a delegate of User A's calendars and has write privileges, User B still cannot make an event or to do private.)
--It allows User B to sync User A's calendar to her iPhone (something that can't be done if User B is merely a delegate of User A's calendar).
In other words, it seems to give you everything that making User B a delegate with read/write privileges for User A's calendar does, and then some. (Sorry that's quite a tangled sentence; read it slowly.)
To keep the calendars in sync to the greatest degree possible, both users have the calendar refresh every minute.
Has anyone seen a problem with a set up like this? Could two simultaneous users corrupt the calendar somehow?