There you go - learn something new every day. I’ve never heard of such a thing in Rhino.
Anyway it’s not important for what you want to do.
All that a 3D printer wants to see is closed meshes with the normals pointing in the right direction (out for outside faces and in for inside faces).
First create your outer skin polysurface. Then shell it for the inner surface and clean up any horrors that appear and join that up into a polysurface. So far so good, except of course the normals on the inner skin are pointing in the wrong direction and as you rightly point out you can’t flip normals on a polysurface. With meshes you can though and you’re going to need a mesh to export anyway, so…
So now make good meshes of both polysurfaces at a density that reflects the 3D printer you’ll be useing. Get the normals pointing the right way useing the DIR command and you are good to go.
Hope that helps, Steve