Simulating multiple vehicles

How can we simulate multiple vehicles communicating via MAVLink messages?

The issue names no files, tests, or entry points and asks how to simulate multiple communicating vehicles rather than describing a concrete change. Start by examining MAVProxy's command-line entry point and vehicle-connection handling, then identify the expected multi-vehicle simulation workflow and a reproducible test or documentation target before proceeding.

Written by the indexing model from the issue text.