I do the same thing, except my second WiiM is in my study and the other one in the living room. They are linked together permanently (easily configurable in the WiiM app) so they are always playing the same thing. Except, the amps they are connected to are not always on, so I can choose to have music playing in only one of the rooms.
In my bed room I have a Google speaker. As I mainly listen to Spotify I transfer the music via the Spotify app to that speaker and back to the WiiMs. Works perfectly in all rooms.