Calculating the Dielectric Strength of multiple materials

[SpiderPig]

If I had two different materials between two oppositely charged plates, would the total dielectric strength just be an average of both dielectric constants with respect to each of their thicknesses? Or is it more complicated than that?

Material A is in full contact with one plate and material B is in full contact with the other plate.

 

[Baluncore]

It is more complicated.
Imagine the layers of dielectric are separated by equipotentials of foil.
Each layer of dielectric is then a single capacitor, one of several capacitors in series.
Work out the values of the capacitance for each layer, based on Er, area and d.
Each will have the same charge, so you can work out the voltage applied to each.
C = Q / V; V = Q / C.
Now you can work out which dielectric layer will limit the voltage of the series stack.