The goal of a trial is to eliminate other possible explanations for the phenomenon in question. You take a large sample, and randomly assign people to groups, s

The goal of a trial is to eliminate other possible explanations for the phenomenon in question. You take a large sample, and randomly assign people to groups, so all the idiosyncratic variations average out. With only one person per group, you can't eliminate many possible reasons for their differing life spans: one may have caught a disease the other was not exposed to, they may have had different nutrition, different amounts of exercise, different habits, a zillion other things. After all, do other twins die at exactly the same time? No. They die years apart for many other reasons. Unless this twin experiment could eliminate all those possible reasons, it shows us nothing about coffee specifically.