Is there a two sided polygon?

There is something called a degenerate triangle; it has 3 sides as all triangles do, but one of them is of zero length, causing the triangle to collapse into a line segment. That would make it in effect a one sided polygon, however if you want to you can consider the line to have a top side and a bottom side (although they are really on top of each other and not distinguishable). So really, there is no two sided polygon.