Thanks @anon35827816
It is making sense to me now.
Because A and B are with floating ground, the potential difference between the two circuits will increase and it wouldn't be a problem if the two circuits are not connected via the signal cables.
As there is no absolute ground(the Earth) in the two circuits, current from the circuit with higher potential will flow to the other one with lower potential, rather than going back to the desired black wire.
I don't believe I am getting everything 100% correct but hopefully the core concept is correct.
Thanks you very much for all of you guys' replies, it did help me understand electricity better, way better than before.