shotgunning might be a bit more efficient (there's probably still some thc in the smoke you'd otherwise exhale), but it's nowhere near twice as efficient. The actualy 'blowback' smoke will have much less than half; the reason it works so well is that you also burn more weed while doing it (that's why the joint is in the person's mouth). So, added efficiency yes, double no.

If you really don't like the idea of putting a lit joint backwards in your mouth, you can skip it; just take a big rip, have the other person exhale, then breathe into his/her mouth. This is also referred to as a shotgun, at least around here.