Just to clarify (as in "Through a glass, clearly") a bit more...

There are two types of "double-sided".
One refers to the situation where each single polygon in the mesh "points both ways". These polygons have abosolutely no physical width. Poser (with either renderer) doesn't handle this type of mesh at all well, which incidentally makes me wonder why Poser still has the "square" prop (which is double-sided) in the primatives library - get rid of it. Only use the "Single sided square" (unless, of course, you like artifacts)..

The other type of "double sided" is where the mesh does have a physical width.

As far as glass is concerned, the mesh should have a physical width.

Hope that helps,
Cheers,
Diolma