Actually I have seen a setup that was quite close to this. Two separate networks, one of them was completely isolated from another, didn’t have Internet access and used a separate set of network equipment. On top of that, the building itself had two entrances - one for the boss and another one for the personnel. You physically couldn’t get from one part of the building to another. It didn’t help the boss though - he was blown up in his car one day. Fun times.
Like two independent internets spanning from your server to my laptop.
Two completely isolated end to end transports.
That's what OP meant when they said you could make it more reliable by having a redundant network. It's just prohibitively expensive.
Then if one internet goes down in any way I talk to you over the other. That's a fairly straightforward fallback algorithm to implement.