The Manager database (including its media database) is stored into the current user's "Documents" folder (so that it can easily be backed up), so it is normal that if you switch users, you will switch databases. However, this can be fixed by changing the database location.
To do that: move the "OnTheAir Manager Local DataSource" folder to any "common place" that both users have access to and then launch Manager while holding the ALT key. A dialog will come up and ask to select the database location. This will be remembered for future Manager launches. It is therefore possible to put the database onto a shared storage space as well.