Assuming your switch is 1-2-off you could just wire them together with either battery 1 or 2, and use that switch position when sitting and using stereo etc. That would be easiest way. If your isolator and charging leads are wired without going through the battery switch you will still charge all batteries when running regardless of your switch position.

Switch reads "Off-ON-Combine"

This is what I have at the moment. I would think the switch would not need to be changed.

Ok, sorry thats a whole different animal. I am also running ACR's on mine but its a twin setup. All you need to do is add your extra batteries to the house battery, nothing else needed. Did you move your stereo/amp feeds over to the house battery already?

Yes, both amp are currenly running from one battery.

So is adding the third and fourth battery as simple as running a wire (proper gauge) from positive side of each battery to both the switch and ACR? And ground wires to grounding block in engine compartment. Or can I go from one positive of battery 3 to positive of battery 4 then split between switch and ACR?

No need to go back to the switch or ACR, from positive on current house battery run to positive on 1st new battery and then to positive on second new battery. Same with negatives or if it is closer you can run them to the grounding block. As long as all + are connected together and all - connected together you are good, no need to run seperate wires back to switch from additional batteries as long as you have a feed going there from one of them.

Gotcha, thanks...should be real simple then. I guess I was reading to much into this.