either connect the positives and ground the one in the back like you would ground an amp
or
get an isolator. you hook the alt output wire to the center post (usually), front battery to 1 terminal and back to the other. you also have to hook your ignition wires to it somehow. you'll get a voltage drop through the isolator though, so you'll never see full voltage on either battery....
but before you do any of that, do you have a HO alt to not only handle the extra battery it has to keep charged, but the system and accessories too??
just connecting the positives is best unless you do a lot of listening with the car off. then an isolator will protect the front battery from dropping and leaving you stranded with a dead starting battery.