Unless it was bag containing an ounce and the whole thing is coated, I don't really see you getting two "good" shots out of it. With the little paper stamp bags, some always gets caught in the little crevices inside the baggies and if you have 30-50 of them you can sometimes get 1 or 2 decent shots out of the scrapings, depending on how vigilant you are about getting all the dope out of them in the first place...I usually empty and rip my bags as I'm doing them so it's not really something I even worry about...
With plastic...I've had plastic baggies that had a lot of shit in them stuck to the edges, and out of everything heroin is the hardest to actually get it to come off, meth is the easiest... I like to take the bag and just rub the opposite sides of the bag against each other between my fingers, that sometimes works...if there's actual visible flakes stuck to the bag a razor can work, but if it's just generalized dustiness you could use the water like somebody said, but then the water sticks to the bag somewhat too...
I really hope that there's actually something in there and that you guys aren't hoping to get high off a single empty bag with a little residue because you might find yourself disappointed...good luck though!