Shadowed Philosopher
You probably want "pretty high", if you're aiming to incapacitate or kill Person B immediately. As for the injuries...they'd be pretty nasty ones, as far as I can tell. Broken shoulders, crushed ribs, possibly broken spine and skull.
Shinigan (Naruto fanfic)
Author in waiting
Don't forget that person A will probably also be hurt by this attack. Likely the force needed to break Person B's shoulders, will also shatter Person A's knee caps.
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Fuzzy Orange Doomsayer
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Depending on the height, A and B will both be incapacitated. B, because he's just had the weight of a teenage boy dropped onto his shoulders from a great height (which might even be fatal, depending on how high he fell from), and A will have both of his kneecaps shattered, though the damage can be minimized if you restrict the height. In fact, if it's too high, A will be as dead as B. It would have to be a pretty short distance, if you want to minimize the damage done to Person A.
edited 4th Sep '11 3:10:33 PM by tropetown
Here's the scenario:
Person A (the average weight of a 16-year-old male) is in the air, distance X above Person B.
Person A purposely falls, straight down, in such a way that both of his knees connect with both of Person B's shoulders.
What value of X would have the highest ratio of damage in person B in comparison to Person A?
edited 4th Sep '11 1:05:41 PM by SalFishFin