Communicating with multiple Espruino devices

Ok - I'd just assumed you might be using the USB connector on the Pico :)

Perhaps you could attach a LED beween 3.3v and B6 on the slaves to help debug?

You could also add:

setWatch(print, BTN, {repeat:true});

to the slaves. If all is going well, the LED should be off normally (data line is high). When you press the button the slaves should transmit some data and the LED should flicker.