I would like to suggest a new intrinsic skill for Alexa, which you invoke with the word "Share". The share command allows the user to play music simultaneously on two different Alexa devices. This is how it would work, using my own situation as an example:
I have an Echo in the bedroom, and two Echo Dots, the "Home Office Dot", and the "Kitchen Dot".
If I'm in the bedroom speaking to my Echo device, I would say, "Alexa, SHARE with my Home Office Dot." Thereafter, and music I play on my bedroom Echo would play simultaneously on my Home Office Dot.
This "pairing" or sharing relationship would persist until the user explicitly invoked, "Alexa, stop sharing."
Any device, an echo or a dot, can be the initiator of the share relationship with the device the user specifies after the keyword "with", as in the example above.