Calculating parasitic capacitance of serial zener diodes
Diode capacitance often rated at some DC voltage such as 0V which decreases  reverse bias.

If it were two fixed caps of equal value the answer you should know for 2 series caps is 1/2 of C as @winny said    

Cs = 1/((1/C1)+(1/C2))

If they were parallel then Cp=C1+C2