1 DB can be included in two Fbs only if one of these FB contains another. Then it's called multiinstance concept. Generally if you have 2 different FB, without any relations between them - you can't make such an assignment.

Also generally you can address absolutely any DB in programm (including DB-instances) like DB1.DBX0.0, but not via STAT area (as i said before).

Think this is short and descriptive

http://support.automation.siemens.com/WW/view/en/18652511