I understand you want the top part of your stacked wall to have a different exterior
finish than the bottom part.
Find out what the name of that top part of the stacked wall is by clicking on the stacked
wall and opening its type properties. In its type properties, click the edit structure so you
can see the "name" of that top part. Then get out of that dialog and go find the name of
that top wall part in your project browser in the list of wall families.
When you find the wall you are looking for in the project browser, right click it and select
type properties. In that wall's properties hit edit structure and change the exterior material
to something other than what your bottom wall material is.