In both cases, the effect of a successful skill check is applied to the first and second 10 feet fallen. Thus, if you succeed on both checks, the second check negates the damage you would have suffered for falling the first 10 feet (but that damage has already been negated by the first successful check, so it doesn't matter) and converts the damage from falling the second 10 feet to nonlethal damage (but again, it's already been converted to nonlethal damage).
