Not possible with just the gear you have now, I'm afraid. If you don't mind the main room speakers being on all the time (and a slight pop when turning the Beocenter on) you might just swap the speakers 1 & 2 around, then the mute function will work on the extra room only.
If you need more elaborate control, you need an external box like a MCL 2A in the extra room. Then you can control those speakers independently, plus remote control the Beocenter from over there.