You can pull any additional video sources from a Video Master (BV8) provided they are connected correctly:
STB
1. Needs to be connected via SCART (or RGB?) as well as HDMI
2. Needs to be controlled via STBC
DVD1
1. Normal connections
BV8-40
1. Needs ML out (like you've already got for BL2000) to link network/BV6-22.
2. Needs RF out - then distributed to aerial in on BV6-22. This is usually achieved by sending the RF out to a central amplifier/splitter then feeding the aerial sockets in all your rooms.
3. Not sure why you'd want to watch the inbuilt tuner of the BV8 in the link room when you've now got a tuner supplied by aerial in the BV6.
4. The main advantage for having a second STB on the BV6 would be to have a second HD source.
Hope this makes sense - I'm sure others will point out any errors!
AP