Caching for directors in same engine

I'm aware of the idea of optimizing cache vs availability depending on how many directors a given host is attached to.  The more directors, the more availability, but the increased chance of duplicate data being cached in multiple directors.

For directors in the same engine, whether just a small VPLEX, or one engine in a medium or large configuration, are there any cache optimizations?  As in, do directors in the same engine have the ability to "see" or access each others' cache (say, via CMI bus) in a way that directors in different engines do not?

Essentially just trying to understand if there is a difference in caching behavior when looking at Engine1 DirA and Engine1 DirB vs Engine1 DirA and Engine2 DirB.  Or if a director is a director, no matter where it is.