You don't want to convect both batteries together during starting. The house
battery will draw from the starting battery ( just like jump starting a car,
only the good battery is in the running car ). If you are separating the
batteries separate the load also. All engin related loads on the starting
battery all house loads on the house battery. Ether get a battery combiner or
a battery isolator ( each has its good and bad points ).

Have a dedicated starting battery with the ability to select the house battery
if needed.

Mike D.
************************

I am redoing the wiring on a small powerboat (6.2 meters). As part of
this I intend to use a dual battery system, I have seen a number of
different circuits that would work. The circuit I prefer users a
latching relay, which is activated by the ignition switch, this places
the batteries in parallel when starting and allows the charging of
both batteries at the same time. When the ignition is off the
batteries are separated allowing one be the house battery without
flattening the other.
At long last comes the question; if the house battery is flat what
effect will this have on the other battery during starting, charging
and the circuit in general.

Thanks in advance
Bruce