When it comes to Bluetooth, physical echos only allow a one to one connection. As I said, an echo stereo pair is an Alexa cloud construct whose audio is delivered from a cloud based Alexa supported streaming service. As a cloud construct, you can’t Bluetooth connect to it.
Best you could do with a WiiM or any other audio source is use a physical cable splitting the left/right signal and feeding each Echo Studio separately via its aux input socket.
Edit: for example, 2 of these to feed from a WiiM Pro/Plus’s RCA L/R outputs, each to an Echo Studio.
If using a WiiM Mini, you’d need to get a 3.5mm stereo to twin 3.5mm mono cable such as this.