I am using (micro)CT data and generating 3-D models using CTAn ( A SkyScan piece of software http://bruker-microct.com/products/downloads.htm)

Basically I am using simple thresholding to create 3-D model STL files.  

What I would like to do is get the number of voxels where the surface area of the two models touch.  

Imagine having a model that is a sphere with another model within it that is exposed on a portion of the sphere.  I want to know the surface area of that exposed portion.  

Does anybody have any advice?  I have been banging my head against tools like 3D-Slicer, Seg3D, ITK-Snap and so on and have not been able to find this capability.