I believe double-sided in the face buttons just disables backface culling; normally, when a face is pointed away form the camera, it is ignored (it is assumed to be on the far side of the model, and thus invisible a waste of processor power). with the culling off, all these polys are calculated, regardless of whether they are visible.
technically, this doesn’t necessarily double the faces calculated, but it’s safe to assume this is the case.
MtheGreat’s method, however, does double the face count (and actually doubles the space the model takes up in the ram as well, I expect), and I would only recommend this if you want a different texture on each side. even then, use it sparingly.
keep in mind that this is all coming from someone with next to no programming experience, so take it with a grain of salt. remember, “when in doubt, test it”