How to find the common faces between to parts of an assembly?

I have two parts A,B.

A-is a body through which air can flow.
B-is the air flow profile ,that flows through A.

So they have common faces when assembled.(whhich are the faces which air touches when it flows).
I want t find these faces.

So essentially, I want to find the common faces of two parts in an assembly.

Thanks
Darvin.

I've not had the time to dig into this particular problem, but I did write some code a while back that finds duplicate sheet bodies in an individual file.
http://nxjournaling.com/content/find-duplicate-sheet-bodies

I think that the code to find common faces in an assembly would/could use a similar strategy.