Currently, I believe your idea will work as long as each dot has it's own account. However, I don't know how you would "multi-room" the dots while they're on separate accounts... Maybe one day this will all be normal, but for now I do believe you're asking a bit much of the one AI.
Mutliple "Sources" on one group is not currently possible. Multi *room* on one source is, and multi source, using their own, speakers, is possible (all at once, only with separate accounts) as I see it for now.
It amazes me how the music functionality seems to be the most asked about features within all the AI systems already... They do so much more without conflict, but music and listener conflict seems to dominate the AI discussion boards of all AI products. No one has had these features unless they're gazzillionaires... yet all the people who buy these Sorry excuses for audio output, want to play their preference when they want it, where they want it and their wife or children want to play their own music as well in even a different place from a different source than the other people wanted to listen from, or all want to use the same source but a different playlist and a different artist... Incredibly complex stuff for the level of AI we currently have available on this Star Fleet Vessel. They're getting there with the AI, but it's not going to happen overnight. And as far as I'm concerned, a lot of folks who can afford this stuff are old enough to remember rotary dial phones stuck to a central wall or desk in their homes as kids (Some even older... when TV was a 18" screen in a 3 foot cube of electronics or a central home radio was the only electronic form of information and entertainment). This AI stuff is AMAZING, yet everyone seems to want more NOW.
OK... end of silly rant, but maybe it will make sense to some of you. I'm just ecstatic that I can turn on/off Lamps and lights at a whim, with my voice. Turning on the TV is a crazy but cool ability that is Super Sci-fi to us old geezers. not to mention changing channels and adjusting volume by voice... we didn't have TV's with multiple sources like available to day... you had 2 antenna choices on the high tech stuff - - cable and 2 wire antenna. Now I use neither. I'm cordless/wireless to the Entertainment center, where all of it is fed by internet sources in my house over WIFI, controlled by IR and Bluetooth which is controlled by yet another layer of controls with the Echo. Of course we have multiple gaming systems too. How cool is this stuff!