I need a command to split two closed surface which are totally nested

In this situation . There are two boxes which are nested completely.The problem is, neither of the parts are solid so there is no intersect between them.I need a command to make them solid or a command to make a gap on the big one from the shape of the surface of the little one.

Objects in Rhino are “Solid” when they do not have any edges that are not joined. The SelClosedPolysrf will select these kind of objects.

In Rhino, there is no concept of having a void area inside a solid (*). That would just be two solids inside each other.

What are you trying to do?

(*) other than a hack which makes those objects useless for all further operations.