Permittivity is measured in Farads/meter. (when in doubt, assume common sense)
For the axial direction, the effective dielectric constant, Dk (or relative permittivity) is the product*sum of each radial area*Dk divided by the total area.
Now if you wanted to compute that in the coaxial direction with two dielectrics in series, that which requires an infinitely small center conductor which is very lossy and inductive (bad idea) otherwise you need to add center hole dimension.