If you upgrade you batt cables, alt and alt cable you should be good. I have done all this on my Jeep and have daul battaries. The biggest change were the cables. You dont want a dead batt off road.
2nd batt. My amps are connected to the 2nd batt but I never leave the stereo on. If i leave a light on or door open the Jeep turn the starter slowly till the 2nd batt kicks in and starts.
Batteries are on an isolator, 2nd disconnects when you shut the car off. Leave anything on and 2nd batt stays charged. For stereo use, 2nd is used allowing starter batt to not discharge. But radio must be wired from 2nd batt too.There is also a kit that allows you to give yourself a boost from the 2nd batt. You can see how thick my cables are.