Dual battery setup

I see where you're coming from in thinking that the current will be split in half, but it doesnt really work that way. Seeing that you'll be mainly using your audio system while the car is running, you havent taken the alternator's power supply into account. Generally the alternator will supply almost all the power to the amps, seeing that it usually gives 13.8v or so. This means that most of the current will still be going down that 4awg wire anyway. If you really dont want there to be too much stress on that wire, you could install a stiffening capacitor. This will pretty much have the same(if not better) effect on the audio system in that it will supply the extra current when the amps need it (pretty much when you hit a large bass note or so). Its also safer for the boot, and they usually have built in volt meters, so u can monitor any voltage drops in the system.

Mind you, the Pioneer company cars here in Australia run 4 500w RMS mono blocks and 2 4ch amps on 4awg. Personally, i think thats pushing it, but unless your running some pretty serious gear like that, i think you can get away with that power cable.

BTW,

Here u go redrazor...this site explains it pretty well...

http://www.bcae1.com/charging.htm

So I would have to run a second line from the alt which also went to the second battery to achive what I want to do?

It would make sense that if you had the room to install two or three 4-gauge wires through the firewall that you would also have the room to install one 2 or 1/0 gauge wire.  If you're only purpose in using two batteries is to split amperage then you're on the wrong path.  I didn't read everything that was written here in this thread, but you must know that amperage starts at the alternator and goes through the chain, to the first battery and then to the second.  Two batteries in parallel is the same voltage as one but twice the amperage capacity.  Having that extra capacity should be the only reason to use the second battery.  It sounds like that is not your intended purpose.

So I would say, find a way to install the bigger wire.

Let me correct something in what i said - it does split the amperage in half, but only when the car is off. Both batteries will be providing the usual 12v or so. When the car is running, thought, thats a different story. The alternator will be providing about 13.8v, and will hence be providing all the power to the amplifiers while charging the (both)batteries at the same time. The only time power will be drawn from any of the batteries is when the power draw from the amplifier is exceeding what can be supplied by the alternator. This will cause a voltage drop in the system and current will be drawn equally from both batteries.