When we are dimensioning throught out a space, say the interior and dimensioning from face of core to face of core, one witness line will snap to the core and the other will snap to what ever it wants. If you hover over the second wall, it can pick up the wall face, core faces and center of wall. If I have core faces picked, there shouldn't be any other option. I don't like zooming in just to discern that 1/2" finish dimension when I am trying to show room sizes for the framers. I figured this out when the temp dims were showing a different number than the perm dims.