Maybe, but it's still trivially easy to break the system. Just slip something in your co-pilot's drink. Or knock him or the (probably female) cabin attendant out with something heavy.
It seems like it's at the same level of the rule that the pilot and co-pilot can't order the same meal at the airport before a flight in case they both get food poisoning simultaneously. Doesn't guarantee that they don't both get sick, but it's a relatively minor imposition and reduces the odds by quite a bit.
