Two identical P-N junctions may be connected in series with a battery in three ways. The potential drops across the two P-N junctions are equal in
In circuit 1 left diode is forward bias and right one is reverse biased so potential difference across both are different.
In circuit 2 both the diodes are forward biased so potential difference across both will be equal In circuit 3 both the diodes are reverse biased so potential difference across both will be equal.