Calculating equivalent capacitance
If you assume V1 to be rigid, changing the voltage of NODE1 needs both capacitors to be charged or discharged as many volts. Thus the effective parasitic capacitance of NODE1 is C1+C2.

More formal proof needs superposition theorem. In AC component calculations DC voltage sources must be set = 0V. Thus C1 and C2 are in parallel, total C = C1+C2.