It is a help to visualize the problem using mathematics instead of words.
Let our friends be Bob and Alice. A has the information set X that B wants to know, and B correspondingly knows Y.
The transaction is complete when A knows Y and B knows X.
We are assuming in this question that if either half of the transaction occurs before the other half, the successful party rescinds on the agreement, and the transaction is incomplete.
This means the conclusion of both information set transfers must be simultaneous, and a simple, reliable method for this is to use a series circuit in proxy.
Once this is achieved, A and B need to validate the information they have received. This requires mutual consent, and again a series circuit suffices to enforce the conditions.