On the expensive side as you'd expect from the marine industry, here's another option for battery monitors:

http://www.xantrex.com/products/product.asp?did=199

However, I find that a good voltmeter can provide plenty of information.  I've just bought one from JC Whitney.
but haven't installed or tried it yet.  For two batteries, all you need is an "a-b" switch, or an "on-off-on" switch so that you can choose to monitor one battery or the other.

https://www.jcwhitney.com/product.jhtml?CATID=175340&BQ=vn

Fin Beven
Pasadena, CA
  ----- Original Message ----- 
  From: David Beierl 
  To: vanagon@GERRY.VANAGON.COM 
  Sent: Saturday, August 09, 2003 3:10 PM
  Subject: Re: Battery monitoring panel needed

  At 04:09 PM 8/9/2003, Gnarlodious wrote:
  >I need a device to monitor the charging and discharging of two batteries
  >connected or separately. Is there a commercially available panel to do this?
  >
  >--Gnarlie

  www.amplepower.com
  www.cruisingpower.com

  Lots of different sites concerning  "off-grid" living...

  d

  --
  David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
  '84 Westy "Dutiful Passage"
  '85 GL "Poor Relation"