Can there be a 2-sided polygon?

It's called a digon. In a flat plane, the two edges would coincide, making the polygon degenerate, but in a curved space it could have two distinct edges and a non-zero area. read more

N dimensions space, There can be a 2-sided polygon but self-intersecting(overlapping)! A 2-D Polygon in 2+ (2,3..N) - D should at least define a NON-ZERO AREA A 3-D Polygon also called 3-D POLYHEDRON in 3+ (3,4,5..N) should at least define a VOLUME. read more

However if you move to non-planar geometry AND generalize the definition of a polygon to"a closed figure ,consisting of straight line segments (geodesics) in the same SURFACE" , then you could argue that there are two sided polygons. For example two lines of longitude on a globe would be a two sided polygon in this sense. read more