So the iTwo has two audio inputs. Where ever your sound is coming from needs to be plugged into the iTwo audio input. It sounds as if you are using the BK-7m for additional sounds as well as backing, so you will connect it to the inputs of the iTwo with audio cables. Your FR-7 will be connected to the BK-7m, via a midi connection, so that you can play the BK-7m sounds from your FR-7 Accordion. The FBC connects directly to the FR-7 for control purposes.
I'm not sure how it works if you want sound from the FR-7 as it seems like you would have to connect that to the audio input of the iTwo as well. So you could connect one audio output from the FR-7 Accordion into input 1 of the iTwo and one output from the BK-7m module to input 2. So you will have two audio sources.
