If I was to add second batterie to my system, would I need a batterie issolator or will I be fine without one.

if you're driving daily then you should be fine. get a volt meter to watch the voltage. maybe two; one on each cell. stay save above 12 V at least.

Unless you're sitting and playing your system for long stretches at a time no real need to isolate the starting battery from the stereo battery imo. As said above monitor the voltage with a voltage gauge.

You would only need one if you plan to play car off for long stretches of time

Also if your rear battery is drained and your car is sitting for a long time. Most of us don't unless we have a huge bank of batteries and not the ability to charge them up.