No offense but there is a lot of miss information on this thread.
1st off a battery does play several critical roles in the charging system of a car even wile running. You see an Alt is not meant to put out even 70% of its output (amps) all the time. It will get way to hot and like any electrical component, heat will make the output diminish and shorten the life of the alt. Also lets say the combined amp draw potential of the S10 is 150 amps, at 1st glance a 200 amp alt is all you need right.....NO. You see a 200 amp alt can put out at times even more then 200 amps but when the system is not drawing 200 amps? Now would be a good time to have some kind of storage device to make the system more efficient. One more thing to think about is a 200 amp alt may put out as little as 70 amps at idle, this would be a good time to use that stored energy, aka battery.
2nd In some cases adding a 2nd battery can hurt you, actually it would in most cases. The main reason is because if your alt is only a 75 amp and your car draws close to that (lest say 40 amps) it doesn't leave much amperage for charging battery's. But with a 200 amp alt and you biggest current draw is a stereo ( witch doesn't consitantly draw its rated current) you will not only have plenty of amperage to charge 2 battery's but you should run 2 battery to, again put that 200 amps to good use.
3rd and finally, when it comes to batt isolators, I say this.....
When it comes the stereos battery isolator will make possible to run a miss matched charging systems but at a high cost! A typical batt isolator uses solid state witch has huge voltage drops, meaning a lot of energy is being wasted. Now if you want to "isolate your starting battery from your stereo you can use 2 constant duty solenoids with little to no voltage drop but you will still need a alt that can charge 2 battery's at once
I don't mean to step on toes but half truths or suspicions can really steer people in the wrong direction because you cant see voltage, its vary abstract.