Here's a post of mine from another forum, we were also talking about batteries.
Optima batteries:
Red top - for high but short loads. i.e. starter motor or winch
Yellow top - deep cycle for extended drain, high vibration
Blue Top dark grey case - marine deep cycle, extended drain, long sitting times with no use
Blue Top light grey case - combination deep cycle / starting battery, long sitting time with no use
You can't go wrong with a yellow top in a 4x4.
One of the simplest ways to run a dual battery setup is to run a battery isolator, doesn't get any easier.
There are either 3 or 4 terminals on an isolator.
One terminal for positive on battery #1
One terminal for positive on battery #2
One for charge wire from alternator
If your charging system has an excite circuit, a fourth terminal for the excite wire (as in a Toyota).
The batteries are both charged by the alternator but they can't "see" each other, i.e. one won't drain the other in unequal voltage situations.
Should one battery become totally discharged you can boost it by simply bridging the 2 positive terminals on the isolator.
Battery location doesn't matter, put it where it suits you best, just remember that good cable costs lots of money, and the cable might end up costing more than the isolator. Welding cable is my favourite.
As speedracerbubba says you can also run a marine switch, should one of your batteries go dead, all you need to do is bridge the positive terminals on your isolator, or put in a switch and let it do that for you.
