Starting with two cubes, side by side.

Overlapped:

Rendered with SuperFly (FireFly doesn't do this, but I want to use SuperFly):

There are other objects that cancel each other when overlapping, but this is the one that's driving me crazy right now. Trying to create wall primitives, and want to overlap the patterns (brick) to line them up properly. It isn't really a cancellation, in that you can't see through the black space:

Doesn't seem to matter what materials are used.

Any clues, tips, suggestions?