Items are dropped even if both key pieces are already acquired.
If one key piece was acquired before picking up Filigree Feather, the next time the angel that normally drops the key piece Isaac currently has is defeated, it will drop the other key piece even with Filigree Feather equipped. (E.g. defeating Uriel while holding Key Piece 1 and Filigree Feather will drop Key Piece 2, but defeating Gabriel with the same items will drop an angel item instead.)
If both Angels are fought at the same time, only the last one defeated will drop an item.
Only angels that would otherwise drop a Key Piece will drop an item. This means that those who spawn during the Mega Satan fight, Boss Rush or those rarely found in closets in the Cathedral do not drop items, but ones found in some Super Secret Rooms and those summoned from Sacrifice Rooms do.
If one key piece was acquired before picking up Filigree Feather, the next time the angel that normally drops the key piece Isaac currently has is defeated, it will drop the other key piece even with Filigree Feather equipped. (E.g. defeating Uriel while holding Key Piece 1 and Filigree Feather will drop Key Piece 2, but defeating Gabriel with the same items will drop an angel item instead.)
If both Angels are fought at the same time, only the last one defeated will drop an item.
Mega Satan fight, 
Boss Rush or those rarely found in closets in the 
Cathedral do not drop items, but ones found in some 
Super Secret Rooms and those summoned from 
Sacrifice Rooms do.
Meat Cleaver: Each angel gives an item.
The Stairway: Allows for an extra Angel in the Angel Shop every floor.
Golden Trinket / 
Mom's Box: No effect.
Jacob and Esau: Jacob must be the one holding Filigree Feather for the trinket to work properly.
The Binding of Isaac: Rebirth
