Try the vray two-sided material. There's a tutorial about it too (floating around somewhere)
Simple steps: 1.make your curtain material - (it should be opaque, but if you want it sheer, you can use translucency and maybe a texture map)
2. create a vray two-sided material (from the vray materials window - and make sure it's NOT the SKP two sided), set both front and back materials to your curtain material, check the box for force one-sided, and set your color to a light gray)
3. apply your material to your curtain. (I also apply the original material to an edge to make sure my material doesn't get purged.)
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