Exactly that. Separate sessions give you data isolation. The hard part is capability isolation, like selective collaboration (between multiple users and multiple agents).

My household runs a shared agent on Telegram, my partner and I can do everything, calendar, purchases. My kid should be on a different trust tier, can ask questions but not send emails on our behalf for example. With a prompt rule the kid can just say 'dad said its okay', but with cast the kid's ingress is wired to a permission set that never reaches certain tools.

That's the simple version. The more interesting case is building agents that collaborate across trust boundaries in real time, but that's a longer conversation.