Run two agents side by side and carry messages between them, turn by turn, until they reach a conclusion. Agents in Herdr can't message each other directly — so **you are the relay**: you read one agent's question, hand it to the other, read the reply, and hand it back. This is the interactive counterpart to `herdr-fanout` (one-shot parallel) and `herdr-pipeline` (one-way chain): here the two participants go **back and forth**.
