No, because there is a connection between both of these devices and all other devices on the phone network and the internet. It’s just bullshit and on top of that overcomplicated nonsense no one is going to use.
I'm talking about physically isolated computers connected to separate networks, not connected to the same untrusted network. The meaning of the isolation is that while operator of each network has one class of data (voice/sms vs. the internet), neither has both of them.