Does anyone know if it is possible to add a secondary exchange mailbox while authenticating with your own credentials? IE an admin assistant needs to be able to view their manager's mailbox, calendar, etc.

In Outlook client this is a simple matter of configuring another mailbox on your current profile. As long as you are delegated access by the user or in Exchange you are all set.

In WP8 it looks like you will need to have the second user's password in order to authenticate as them. This is obviously not ideal. Has anyone found a way to get this working?