The way you describe your accounts and devices is what I'm trying to get mine to do. I have two people on the account and even though all devices, no matter what I do, are registered to the person to which you're in their account. Switch accounts and they all are registered to that other person. I want and think you should be able to have two complete individual sets of devices and connections, but in the one master account. Especially since one of the set ups has an Echo Connect and the other does not need it. All I really want is to be able to "Intercom" between all the devices.
I have read through a ton of Amazon stuff and it implies that it can be done and what you are looking to do also. But no distinct directions as to how to do it.