So in the first step, when Alice sends a message to Bob, and Bob sends a message to Alice, they both use the secret s_B and the nonce 0! In the example above, Alice and Carol coincidentally always had different nonce parity, and you could make that deliberate: in each channel, you have some way to designate one party as “odd” and one party as “even,” and then, to send the message with the nonce n, you actually use 2n if you’re the even party, and 2n+1 if you’re the odd one3. However, the actual threat of nonce reuse doesn’t care who sends the message—and if anyone sends a message with the same key and nonce, you’re at risk.