Todd is right. Think of a starter relay between the two positive posts of your two batteries. When the battery voltage increases because there is a charge source applied, the constant current contractor closes and ties the batteries together. When the charge source goes away the contacts open. The source can be your engine, your solar panels, your battery charger or any other. The advantage is no voltage drop and a better, more accurate sense for your smart charger(s). The control of all this is a simple voltage relay, usually with adjustable high and low set points. It usually comes with a way to attach a button on the dash so that when it's pushed manually it closes the contractor in an emergency for starting if your start battery is to low.