How to set up phones & Bluetooth on Columbus infotainment

My Superb (17 plate) has been fine with 2 phone users for years - but now we increasingly  use phone apps to listen to music, rather than the radio..

And here comes the problem - the car seems to choose randomly whose phone it will connect to for phone/Bluetooth music! And the same for whose phone contacts it will download.

So is there a way to force the car to choose a particular phone for phone calls and a particular phone for music via Bluetooth?

I've tried the documentation - of course - but it isn't really obvious, at least to me!