The only possible problem with your idea is if you run down the main battery it
will over load the small extra battery. There is lot of loss with jumper
cables plus if you can't disconcert the dead batteries they pull a lot out of
the good battery.

Not to mention the probably sparking during hook up. Problem never happen in
good conditions. At least for me anyway.

Mike
***********

I would echo what others have contributed about not routinely drawing
from the house battery for starting if you set up as you describe. What
surprises me is that almost no one selects a setup I prefer: a
large-ish bank of house/starting batteries, without switches, relays, or
problems. "What if you drain the batteries down?" For this purpose,
and for this purpose _only_, I bought a cheap starting battery.
Unconnected to anything. Once a month, I would affix heavy gauge jumper
cables from it to the main bank during a 6-hour motor to re-charge the
self-dischaged capacity. Left the cover off the battery compartment
while under way so there was no way to forget to remove the cables
later. ("Gee, why are the batteries visible from the saloon?") Cables
served for the emergency start, if needed, connected directly to the
starter solenoid.

HTH,
Rich Mechaber