You may want to have a long plane, all to be able to look at its both sides.
You'll see that only one side is visible.

Why?

Because Sandy3D is using an algorithm, backface culling, which allows to avoid computation and especially drawing, any polygon which is not facing the camera.

When you define a polygon, you define also a normal vector. This normal vector will be used for the backface culling phasis.
After being transformed, if the normal vector isn't facing the camera, the polygon is set as non visible.

If you want to go through this, you need to disable the backface culling like this :

myObject.useBackfaceCulling = false;

Mind to apply an a specific material for the backside to the appearance if you expect it to be different from the front one.