An FBC module is a complex. It has two physical connections. They can be interlinked/internally connected like some non-FBC tuners, they don't have to be. It's your choice.
It has 8 demodulators. 2 are the physical connection ones which can be shared), the other 6 are what you decide to share with a physical connection.
You can hook up a unicable LNB to tuner A. You can hook up a normal/universal LNB to Tuner B. Or other way round. You can also use diseqc switch, positioner, USALS motor. Just like a normal tuner.
The golden rule of tuner config is that it must match what is hooked up to the tuner.
With regards to sharing out to C-H. If you share out a unicable connection, it will be an independent demodulator (barring processes that need RAW tuner). If you share out a universal LNB, it will be loop-through.