In end-user accounts, more than one user can have the same phone associated to their account. In classes, I give the example of an executive who wants a secretary to be able to control some of the functions of an executives phone such as setting up a speed dial or changing the target of single number reach.
I'd have to test to see if that means that two different personal directories could be accessed from a single phone. If not, perhaps you can use Extension Mobility with the same primary DN on two profiles. Those would be separate objects in the database, each unique to one user.
