electricity - Breaking up a capacitor

Suppose there is a parallel plate capacitor of area $S$ and separation $d$. Two different dielectrics fill-up the separation between the capacitors as shown below:

; where $\epsilon_1=2$ and $\epsilon_2=4$ are relative permittivities.

To find its capacitance one can break up this capacitor in two ways:

The respective circuit diagrams:

The first break-up gives capacitance $C = \frac{12\epsilon_0 S}{5d}$, whereas the the second break-up gives $C = \frac{7\epsilon_0 S}{3d}$. I do not understand which break-up is correct and which one is wrong and why?

Answer

When you have a dividing line which is between two dielectrics parallel to the plates you have to ask yourself; is the dividing line an equipotential?

That is relatively easy for diagram B as there is no change of dielectric on either side of the dividing line.

In diagram A there is a change of dielectric on either side of the dividing line and so in all probability there is a change in the potential along that line.

To check what I have written use diagram B find the the potential between the two dielectric.
Now imagine continuing that dividing line into the bottom dielectric and find the potential along that line.