Quote - Hrm the program only gave me one side.

A polygon, by definition, is one-sided.
A 2d plane (w/ no segments) is made of two one-sided polygons...front and back.

Quote - Can I define the front and back of polys as different material groups?

No. There is no back to a polygon.
You should definitely be able to assign different materials and UV maps to the opposite sides of a surface; because the two sides are not the same group of polygons. I don't know how you would do that with C5.

Quote - Does that depend on the program Im using?

Sort of. I have shaders that make one-sided polygons two-sided and individual materials can be applied to the opposite side. But; that is really making two polys out of 1; certainly at least in terms of render time hit.
You also can't have individual UV Map co-ords doing that, so you are forced to use the same UV co-ords on both surfaces, AFAIK; which isn't always desireable. It is better to have the two poly surfaces to work with in the first place a lot of the times.

Quote - Does this mean that I have to make all things with two layers of polys?

Yes. If you want the backside of a surface to be visible, then you are in effect making another polygon group.
It is a complete waste of resources to have non-visible polygon surfaces; unless there is transparency involved (when you could see the hidden polys) or the object is made to be viewed from all sides. A floor, for instance, only needs the top side; the bottom polys are a waste of resources.

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