There’s no such thing as “flat.” We call things flat because they appear to have no depth, but everything has depth, otherwise it couldn’t physically exist. Everything, no matter how seemingly “flat,” has three dimensions.
Consider a sheet of paper. Flat, right? Nope. It’s depth might be microscopic, but it’s there. How can we tell that something has depth? Easy: something has to be separating the front side from the back side. If there are two sides, there must be something between them, some point at which one ends and the other begins. Therefore, a piece of paper does have some tangible thickness in its edge.
So if anything that has two sides must be three dimensional, conceivably, any truly flat object–any truly two dimensional object–would have to be transparent from behind. No depth means no back side allowed.