i believe-i could be wrong- all THC is abosorbed on impact.
meaning that as soon as the smoke touches your lungs, it swells and closes the capilary's and blood vessals in your lungs, leaving a small precentage of thc left in what you exhale, so for shottys from mouth to mouth, that would explain two people getting high.
now i know holding it in has nothing to do with the price of tea in china, simply because i used to hold my bongrips for an unreasonable ammount of time when i first started off- and guess what, i got a headache.
now after several years of this rediculous method, i finally got my own glass on glass bong and let me tell you, as soon as the last bit of smoke slides its way out of the top of my bong, im exhaling as soon as my mouth is clear of the chamber, and i still get quite ripped and i seem to get a more pleasant "after taste"
so my point, i inquire you all to stop holding your bong rips for a week and just try letting it out right after you inhale. see how it goes for ya, i mean...whats the worst that can happen? i promise so long as you inhale, you can exhale immediately there after.