I’m in the opposite position, I only have one Naim streaming device but the app has the multi-room icon. I only get to see it on the Play queue screen and it is a little icon with 4 squares in the bottom right corner of the screen
You only see the multiroom icon on the Now Playing screen. So start one player playing, tap on the bottom bar and that gives you the Now Playing screen and you should see the multi room icon. Tap it and you are good to go.
The other thing to know is that you can’t multiroom high resolution music in first generation players.
Best