Theoretically no, though it might be close (have no idea exactly how close it would be). The extra mass of the fat guy lets him oppose the air resistance a little better than the skinny guy. If air weren't a factor, they would both hit the ground at the same time.

Edit: Now that I think about it, maybe the skinner guy is more aero since he has less of a profile. So maybe the skinny guy will hit first. Anyway, the point is that air resistance complicates things, but it's not the reason why the average mph in the problem in the OP isn't simply the average of the uphill speed and the downhill speed.