I mean, it's still the same network. They show up as different options in your wi-fi menu, but devices can still talk to each other on 192.168.0.x, right?

Most (?) wi-fi routers now support 2.4+5, and it's not meaningfully more expensive. I assume wifi 6 will have the same kind of gradual introduction 5 had?

I've never heard of anyone buying separate routers for multiple wavelengths.