I am not familiar with the VW setup, but my van uses a battery isolator to
automatically charge each battery when needed.  The isolator acts like a
diode by preventing current from running back out of the batteries into the
charging circuit.  The problem of using switches (without an isolator) to do
this is you can get competition between (dueling?) batteries for the charge.
One battery will "charge" the other and they can switch back and forth
draining both.  My suggestion is to get a battery isolator ($30+).  These
come in versions for two or more batteries.

Yours,

Rob

http://msnhomepages.talkcity.com/PassportPl/gunter01/