You can place different BS5 in a masterlink system. But they will all have a BM5. Note the fortcoming BS5 Encore doesn't have ML (It is rumoured that B&o is working on a kind of solution).
That doesn't mean you must have multiple music collections. Choose one as master place all your files on this master. Set this one up as music server (see set-up menu)
Now you can have simultanious multiple connections with this music server.
In the link system you have the possibility of five commands Radio, CD, A.Mem, N.Music & N.Radio. So I presume if the main BS5 is controlled with f.e. CD & Radio, locally you can choose N.Radio & N.Music for the local BS5 and CD, Radio for the main BS5.