Think of it as two caps in series, with a virtual plate at the
material interface. 1/C = (t1/e1) + (t2/e2), so
(t1+t2)/e' = (t1/e1) + (t2/e2)
e' = e1*e2*(t1+t2)/(t1*e2 + t2*e1)
Of course, this only applies if the layers are close together
relative to the size of the conductor; if the space near the
conductor is dominated by one material or the other (eg a
75-micron trace with 150 microns of FR4 and a few light-years
of air and vacuum above it) then the farther dielectric
has less effect.
-- D. C. Sessions [email protected]