I have the same wiring on my Jeanneau (same parent company as benes).
The house goes to the house bank and the engine to the engine battery.
There is a combiner factory installed which allows the alternator to charge both but otherwise, the two circuits are completely separate.
I kept the negative switch as is and instead added a 'jumper' red cable connecting the load sides of both the house and engine banks with an on/off switch in the middle. It was just easier than re-routing all the negative wires to single bus.
The effect is the same. I can run the house off the house bank and the engine off the engine bank. Should either one of those fail, I can run the engine off the house bank and the house off the engine bank. (Or I could combine both banks, not that I would ever want to do this).
(the white wire in the pic is my solar which is connected to the house bank via the controller mounted just on top of the balmar smartgauge. Once or twice a year I manually switch the lead to the engine battery just to top it off, like just before winter storage)