Warren (Official Rep) 1 month ago wrote :
"If you have more than one AirPlay device in the same wireless network, please try the following:
Open “Audio MIDI Setup” app and select “Show Audio Window” from the “Window” menu.
Click on AirPlay in the left tab.
In the right tab, there should be a dropdown option where you can select the desired AirPlay device."
but I’m also asking :
“What shall I do if I want to display on all my Airplay devices at the same time ?” (which is I guess the basic aiming of multirooming… )