Two simulators
Run the app on two simulators at once, see both in the window, and tell the agent which one to use.
In the window
Switch the toolbar from One device to Side by side. Two simulators show as two columns, each with its own live screen and device buttons. The Device panel has a scope menu that chooses which of the two devices its Inspector, Network, State and Navigation tabs show. The simulator picker in the toolbar chooses which simulators are shown; choosing never boots one.
For the agent
With more than one simulator booted, the device tools (device_tap, device_ui, device_type, and the rest) need a device: a simulator id or name from device_list. Without it they answer with an error that lists the booted simulators.
With more than one app connected to Metro (the same app on two simulators), the app tools (app_tree, app_logs, app_errors, app_commands, app_command_run, app_reload, app_data) need device too. It can be a simulator id or name, or the target id the error lists when two apps report the same device name. Metro reports the device name only, so an app is attributed to a simulator by that name. With one app nothing changes.
The window's selection is never used for the agent's calls
Which simulator is selected in the window does not decide which one an agent call goes to. The agent always names the device itself.
app_reload asks Metro to reload every connected app, so it also reloads other devices on the same Metro.