I have installed many 2 batt syst. in my own and customer vans and cars.
I use a constant on solenoid that is used to run the light bar and other
elect. equip. on our Police cars. it is also used in bus's. Napa part #
ST85. It looks like a Ford starter sol. but is all steel and made to be
on all the time. (not just to start a car as in a standard starter sol.)
They are rated over a 100 amps. I have had this setup in my Blazer for
over 2 years and run 2 900 CCA batt. and a 100 AMP alt. And in my Westy
with 2 825 CCA batt. behind the seats and a 3rd 825 Gel cell under the
rear seat, with a 90 AMP alt. for over a year with no problem. I always
up grade the batt cables and wiring.    Bob K.  RWK Enterprises
Placerville Ca. VW Mods and Conv.