Jens:
you could probably do what you are after, with less costly relays
or starter solenoids rather than the expensive series/parallel switches
i havent bought on in over 25 years and i am not sure anyone uses them anymore
and iirc the smaller 2 post unit was 200bucks and the larger 4 post isolated version was about twice that.
as opposed to 10 dollar ford starter relays and a few other small automotive iso cube relays, some logic control
and you could probably do what you want for a small fraction of one of those delco units
i used one of the 4 post units because i needed "dead" leads until i hit the remote switch
and needed a way of charging both halves of the bank on 12 volts supplied by the service van
gotta be a cheaper way, mainly because you don't need "dead" leads.
bob g