Mark,
I agree some certainly claim to have had no trouble.
I had a deep cycle marine in the drivers battery compartment.
I used a large main battery lead from a wrecking yard for the positive and connected it to the starter alongside the man battery lead.
It worked fine but after about 2 years they both gave me problems.
It may be one went bad and it pulled the other down and it went bad as well.

It just did not work out well for me.

I have found I never really used the extra capacity of the 2nd battery much.
I think I can get away with a good main battery and just don't over drain it.
Less is best.

Doug

  ----- Original Message ----- 
  From: mark drillock 
  To: vanagon@GERRY.VANAGON.COM 
  Sent: Monday, June 07, 2004 6:18 PM
  Subject: Re: Dual-Battery Question

  I have put dual batteries into many VW vans and had no problems with
  battery life. Hooking 2 together is a simple way to get more capacity
  for camping while still having plenty for starting when it is time to
  leave. A manual switch or proper relay is an even better way to do this.
  I have left the 2 stock size batteries connected together in my 87
  Camper for about 10 years now. I don't see any truth to your concerns.

  Mark

  Doug in Calif wrote:
  >
  > Fin,
  > I have found the two batteries will fight on another and deteriorate.
  >
  > Any un equal difference will constantly pull on one and push the other to hold more which eventually made them both go bad prematurely..
  >
  > Doug
  >
  >   ----- Original Message -----
  >   From: Fin Beven
  >   To: vanagon@GERRY.VANAGON.COM
  >   Sent: Monday, June 07, 2004 5:14 PM
  >   Subject: Re: Dual-Battery Question
  >
  >   For years I have linked my two (identical) batteries with a solid run of 4 ga. wire, with crimped-on terminals at both ends.  This effectively turns the two batteries into one LARGE battery.  What's critically important is to use VERY high quality wire and terminals.  If you have access to marine grade (West Marine on the Internet) I think that it's worth the price, as they assume that their equipment will be used in a very harsh electronic environment (salt air).
  >
  >   This is not a recommendation, but no problems have resulted.  However, I do have a good quality volt-meter that can be left on when the batteries are being used but the engine charging system is not (as when camped overnight).  I'm just cautious not to let the voltage drop below 11.8 in moderate weather.
  >
  >