With multiple insulation systems in series, it is NOT as straightforward as Rtotal=R1+R2. Instead it depends on the relative permitivities and the thicknesses of the layers. For two layers we have:

V1 = V/(e1*(d1/e1+d2/e2))

where V1 is the voltage across layer 1, d is the total thickness, d1 is the thickness across layer 1, e1 is the relative permittivity of layer 1 (air=1.0), and e2 is the relative permittivity of layer 2. Similarly the voltage for layer 2 is:
V2 = V/(e2*(d1/e1+d2/e2))

This can be generalized to as many layers as necessary.

Typically where this is used is to show for instance that wrapping extra layers of insulation "for good measure" can actually be detrimental in increasing the voltage stress to the point where the "extra" actually causes a failure. In a similar way when combining say rubber gloves with a hot stick, the resulting voltage stress across each of the layers is not necessarily predictable and may result in a failure where only one insulation system may have been fine.