Hi, I'm the OP
To the other part of my question... Assume there is another hill after the first, the guys are still coasting, would the lighter guy catch up on the way up the hill or come to a stop further up the hill than the heavier guy (or the other way round)?
As for the coasting uphill part, I honestly don't know. Assuming a higher speed at the bottom of the hill by fatso, I will guess that he would coast a little farther up the hill, but, I'm not as comfortable with this assumption as with the downhill part.
If they are pedaling the uphill side, I suspect fatso will get dropped like a hot potato, assuming comparable fitness levels.