One who carries a heavy object outside on Shabbos is liable. If two carry an object that either could have carried alone, they are exempt. If they carry an object so heavy it requires two, they are liable. Rabbi Shimon disagrees. Key concept is that one most perform an entire transgression to be liable, not just part of one. Related cases include relying on the mistaken ruling of a rabbinic court, combining one who is capable of carrrying alone with one who is not, a zav sitting on a bed or a donkey, a priest performing the holy service with only one foot on the ground in the Holy Temple, and two who combined to steal an animal, but only one slaughtered it.