My brother has a duel battery with one as a designated starter and the other for all of his aux. electrical. He went with an isolator to do the job of charging when needed. Check out the "painless" site:

http://www.painlessperformance.com/...partsearch=isolator&submit=Go%21

A simple phone call to one of their reps. will give you tons of info. about suggested components for your situation.

To monitor the battery charge he installed a vdo gauge with a toggle that can switch between each battery.

Hope it helps.

'02 Montero: OME suspension, ARB front bumper, homemade skid plates & spare tire Blitz can mount, extended diff. breather, K&N air filter, Optima Yellow Top, BajaRack, IPF lights, Goodyear Duratrac 265/75/16's, RUD 4x4 Grip chains