Unless u want to listen to ur car audio when engine is turned off OR want to run an amp by battery, a 2nd battery is useless. The alt. supplies the power to the amp when engine is running. Besides if u used a 2nd battery u'd have to charge it pretty much every nite and only run 12v to amp
wiring 2 batteries in parallel results in a generally larger and more powerful power source. the voltage stays the same (12v), but the amount of current that can be pulled out of the 2 batteries is increased. but in a car- the alternatior would have to work 2x as hard to fully charge both betteries...i personally woulndt do it unless i though my alt was strong enough.