If both of the 'borrowed' circuits are dead there will not be an imbalance. As soon as current flows then one or the other (maybe both) will trip.
Unless there is an Neutral-Earth fault taking some of the neutral current from active circuits. The circuit for this is from the neutral bar, ""backwards"" through the RCBO sensor onto the neutral of the dead circuit, through the neutral-earth fault on the dead circuit and then via the CPC to the neutral at the cut out where CPC and neutral are connected.