Two-way rules can also be said or presented in different ways. Again the form of a rule does not matter, provided we recognize exactly what is meant. This form suggests we call such rules biconditional statements. The prefix bi- here signals two ways. You may prefer to say if and only if instead of when and only when.
Tone provides the only difference between the two suggestions.
Both of these suggestions represent a two-way obligation to which we might agree. Confusion or disappointment or false expectations may happen when suggestions such as these are not explicitly accepted or rejected. Two situations or conditions, each of which must happen whenever the other does, are said to be equivalent to each other. So when a first situation is equivalent to a second, each situation implies and is implied by the other. They are interchangeable.