>  With this setup is there any way to isolate the MAIN battery
> so the aux could be used for starting? If the main battery is somehow
> drained and then put in parallel with the aux the main would draw current
from the
> aux making starting that much harder.

Since the main battery in this setup is used solely for starting (barring a
failure of the relay, in which case its intended operational method would be
irrelevant anyway), the only way you could kill the main battery would be by
continuously cranking the starter until it died. If this occurred, you would
have a problem unlikely to be solved by a second battery anyway.  The best
way to ensure that you will not kill your main battery is to make sure that
no accessories are running of of it.  This is easy to do.  After you think
you have wired everything correctly, disconnect the starting battery with
the key off.  If any of your accessories shut off, they are still running
off of your starting battery.  Conversely, if you disconnect the aux
battery, all of the accessories should shut off; if not then either they are
running off of your starting battery, or your relay is malfunctioning or
improperly wired.

- Ron Salmon
  The Bus Depot, Inc.
  http://www.busdepot.com
  (215) 234-VWVW