Mirrorring from different boards

I think this should exist, but I am unable to create it. Does anyone have any idea?
For example, i have 4 boards.
Board A, Board B, Boards C and Board D.
Board A contains all details of contacts, let’s say john and his phone number
Board B contains all details of companies who use our services, let’s say a supermarket and their bankaccountnumber
Board C contains all of our projects, which mirrors John and his phone number from board A, aswell as his company, the supermarket. It is undesirable that all of our employees see his bankaccountnumber.
As soon as all work for John and his supermarket are done a new item automaticly is made on board D, our finance board.
It mirrors from board C: Johns name, his phone number en the name of the supermarket. Now I want it to automaticly mirror his bankaccountnumber from Board B. Which should be obvious since Board C already is connected to the “supermarket-item” on board B. I can’t seem to get it to work. The only way I see this working is mirroring every detail on board B on board C, so I can mirror eveything on board D.

As you can imagine, I’m not to eager toward that solution.

I’d like to hear what clever ways you can think of.