In Rhino 7:
A. Go to Rhino Options > View > Display Modes > Shaded (*) > Visibility
B. Uncheck: Show Tangent Edges, Show Tangent Seams
This will remove any edges that belong to separate surfaces that are coplanar (like your 2 box example, even if they are separate objects) or any seams that are tangent (like the seam of a cylinder, seam of a sphere, of two tangency matched surfaces, and so on).
note (*): You can replace Shaded with any mode you want to hide tangent edges/seams.
edit: this works only with VisualAQR Walls. How can it be? They are separated objects as well.