Wiring Two Batteries in a Parallel Circuit
If you would like to use two 12-volt batteries in your electrical system, than there is two ways to hook-up the batteries - in series or in parallel. If you choose in parallel you retain the 12 volts, but increase the available power. People who configure their vehicle's electrical system like this usually run high-draw aftermarket audio and video system equipment. When these systems are not in use, the extra battery is getting charged just like the standard one. When extra amperage is called for, the second battery can supply it without burning up or short-circuiting the stock electrical system.
These vehicles may also make use of high-amperage alternators or have multiple alternators. They will also probably include a 1,000+-watt amplifier to help supplement the vehicle's electrical system without overloading it.