Hi, I have three dots around the house all connected to ONE amazon account with music unlimited.
It seems to me that you can only stream music to one dot at a time.
Ask her to play, deep purple on one dot and she does, go to another dot and ask her to play adele, she says music is streaming on another device, do you want to stream to this one instead? so no you cannot stream music to two different dots BUT alexa play radio 4 on one dot, Alexa play Bruce Springsteen on another and she does both I also think there is something on the type of account you have, I think you can get a family account that is more money but does allow to stream music on different dots.
Actually just done another test and I think its more down to control, when she asks if you want to stream to this device and you reply yes she will play, but you then cannot ask he to skip to the next track on the original dot, which is why the radio will play without any issues as you cannot skip a radio track.
So Yes I think you can listen to different unlimited tracks on different dots but you loose some control on one of the dots.
Hope this helps
Hope this helps