The Federal Flight Deck Officer page on Wikipedia says this:
Under the FFDO program, flight crew members are authorized to use firearms. A flight crew member may be a pilot, flight engineer or navigator assigned to the flight.
To me, it seems like this would be crucial information for the PIC to know, if their flight engineer (for example) was armed; but on the flip-side of this, the engineer might want to keep that to himself if he's with a crew he hasn't flown with before.
Is there a guideline on whether an FFDO should inform the crew that he's armed?