Slave mode which is audio option 6 (or is it 5? i get so confused!) is intended for use where you want a second hifi in a link system which will only operate in that one room, it will not get over the fact that both systems will respond to the same CD, RADIO commands if in the same room.
Here's a theory. If you have a B&o TV with masterlink you could connect the beosound 9000 to the tv using masterlink, the beocentre 2 to the tv via scart, and make a connection from the twin phono out of the beocentre 2 to the 7 pin din aux connectin of the 9000. Put the TV in v opt 2, the 9000 in a opt 0 and the beocentre 2 in a.opt 0. The beocentre 2 will only respond to DVD command via scart, the 9000 will respond as normal, If you want digital radio you will have to switch on the beocentre2 using its master unit, select a staion, and then select A.Tape on Beo 4 to open the aux in of the 9000
Hope this is useful
Beocentre