how to render double sided faces

There is almost no saving of "rendering overhead" when you disable backface culling instead of modelling both sides manually.

Only if you don't need any lighting. That means no shading, no specular highlight, ... just a solid color. In this case you can turn off backface culling in the shader so both sides are rendered the same way.

If you need shading / lighting you either have to:

• model the other side as seperate face. (This is the normal, most simplest, most versatile and most used approach).

• use a two-pass-shader where pass one renders the front side (so it does backface culling) and pass two renders just the back side (with frontface culling). The second pass would use the inverted normal vector.

Both of those two approaches have some advantages and disadvantages:

 Seperate backface                                Two pass shader
 -------------------------------------------------------------------
 + You can adjust all aspects of the face     |   - Both sides have to use the same
   manually. You can use different uv-coords, |     Vertex properties. That means
   normal and vertex colors for each face     |     textures would be mirrored on
                                              |     the backside.
 - If you have a lot double-sides faces you   |   + If you only have double sided
   need a vertex / index buffer with about    |     faces you can store more faces 
   double the size.                           |     in a single mesh
 
 * In both cases the computational overhead is almost the same, since the seperate
   backface requires an additional face to be rendered and the two pass shader will
   render the same face twice which results in the same overhead.

That's why in general it's more convenient to have the backface as seperate face since it allows you to use custom UV coordinates, normals, vertex colors, ... for each face.

The GPU has a rendering pipeline. It's optimised for doing the same operations on huge amount of data. So the saved amount of data when using a two-pass shader is only a marginal gain over the disadvantages that comes with that approach.

In case you don't need any lighting (and in case you use textures it's fine that the backside is mirrored) you can simply turn off backface culling in your shader like mentioned by @watermy above.