Revit Curtain Walls - Grid Selection

[avb124467]

If you have two curtain walls parallel to each other and you cut a section between the two to look at only one of them. Without any possible way to see the opposite curtain wall in that section, why am I able to pick the grid lines of the curtain wall opposite of the one I am looking at.
I have found that I have to tile my views and work in a 3D window in order to make sure I have selected the correct grid on the correct curtain wall.

[Dimitri Harvalias]

Any chance there is a door in one of those curtain walls? When Revit cuts a section through a family it tends to find the ref planes from that family. So the door, or the curtain mullion profile, may have a stray ref plane that intersects the section.