Best answer by MikeV
You would need two Amazon accounts, so each Amazon account can be associated with a different Sonos household and the appropriate Echo Dot device in that household.
Alexa is more than just a Dot in a house. The system is entirely cloud-based... you could even send commands through the Amazon app while you're not at home and it would still work. Since the brains of Alexa aren't on your home network, it doesn't know where it is. It just knows it's on the internet.