I have not done via an Apple TV but have used Airplay 2 via iPhone. You have to enable Airplay 2 on both AxiomAirs to get it to work, then select both of them on your iPhone under Airplay 2.
That's a great point, thank you.
Also note that if you have not updated to the latest software yet (v2.12.4) you should consider doing so. Airplay 2 is still in beta, but it will now remain active after a reboot or power off.