For the methods I have given above, I think, it's the most appropriate methods for exported models. Of course that support SH3D Both Sides parameter in DAE files.
But to understand why this method is the normal method for sketchup, I invite you to visit this link which explains the inversion problem of faces in SketchUp. (In my language Fr, sorry)
http://www.arch-image.com/wordpress/2010/08/2...facefront-et-reverseface/
Personally, at the beginning of the Beta, I realized the problem faced when rendering in Sunflow; I then solved the problem by texturing the two sides. I just wanted to understand or was the problem. Especially since the two sides texture made me lose time. Actualy, so I put the faces in the right order, and I chose to export only the visible sides to win on the file size. This gain allows me to select triangulated faces to assurate no problem when importing to SH3D.
joke: a dusty lamp illuminates less. You change the light bulb with a higher power light, or your clean the lamp?
ok. it is no one there. I took the door. 
