That's how mine is setup up, I installed it in 2004 if I remember correctly when the battery was my AV was almost 3 years old.
I have a dual battery wiring kit from Wrangler, the Painless kit is pretty much the same. It has saved me from calling a tow truck for a jump or flagging down someone a few times when my AV has been sitting for a few weeks without starting.
How it works and wired (crude drawing) plus Big 3 plus..
Position 1 on the switch - Battery connected when ignition is turned on, both batteries charge when running. Batteries disconnected from each other when ignition is off and anything connected to aux battery does not drain the main battery.
Position 2 on the switch - Batteries isolated/disconnected from each other only main battery chargers when running.
Position 3 on the switch - Batteries connected ignition on or off, both will charge when running, both will be used for whatever load is connected to either. In this position is also how to jump start yourself if your main battery is discharged providing you do not have load/drain on your aux battery.
Edit.. I also installed a Powermaster 225 amp alternator and upgraded the charge cable from the battery to a 1/0.