I don't know PBTR, but in other raytracers the trick is to create a single surface between the two materials and set the ior of that surface's material to the quotient of the iors of the materials o both sides.
That way the light will refract on the internal boundary as if it moves from the one material to the other.
Prerequisite is that you ened to be able to create non-manifold objects...
I have to figure that one out every time I do it ;-)
The resulting interface ior should be positive if you go from a less dense medium into a denser medium, so I guess the material you're going to goes on top.
(which matches what happens from air-> glass. ior air is more or less one, mior glass = 1.5 so from air to glass -> ior 1.5)
That way the light will refract on the internal boundary as if it moves from the one material to the other.
Prerequisite is that you ened to be able to create non-manifold objects...