..unless you have a mil. spec truck, then the only was to go is to wire them in series lol
anyway for the whole relay thing here ya go:
"Easy way to control your dual battery current.
These Painless Performance dual battery current control systems include a three position switch to control the second battery through the operation of the solenoid. Switch position one isolates the second battery. Position two connects the batter while the ignition is on. Position three connects the second battery at all times."
Painless kit 100 bucks from Summit
http://store.summitracing.com/partdetail.asp?autofilter=1&part=PRF%2D40102&N=700+115&autoview=sku
If I were you in a 12v vehicle for your purposes, unless you like to sit in your car w/ the engine off, I would just wire them in parallel and call it a day.