Hi Kiran,
If you still want to avoid the double-volume issue then you could still use the BM7000’s Line-Out. This shouldn’t effect use of tape; both Tape 1 and Tape 2 DIN sockets should provide input and output at the same time.
However, there is another way of looking at this. You effectively have two B&O devices sharing one ‘set’ of Powerlink speakers. That is exactly what this device is designed for: https://shop.oneremote.dk/shop/69067-powerlink-switch/4162-powerlink-speaker-switch/