The one I meant has to do with the killers' and victims' health habits.
In "Exercise," the killer is a health expert and the victim eats "slow poison" take-out food and drinks too much coffee, which the killer has a low opinion of.
In "Double Shock," the two victims are likable "health nuts" and one of the killers cooks gourmet foods, at least one of which gets "a big nutritional F."
So when it comes to the killer's and victims' habits, almost everything about the first one gets reversed in the later one.