You can "just parallel the batteries", but then both will act as one
big battery....i.e. if you leave something on and forget to turn it
off (after a day hiking etc) you will not have enough power to start
your engine (unless you have a m/t and are parked on a hill etc).

IMHO, the best solution for two batteries in a Vanagon is to use the
Surepower model 1315 battery separator.  (do not use an isolator,
especially with Subaru engines).

The 1315 has been discussed at length in the archives.  It's
expensive (around $75) but a really elegant solution....

>I would like to understand; why can't you just parallel the batteries;
>why do you need relays and stuff? Is there a schematic kicking around
>somewere?