Extra battery in trunk....

I'm not too big on stereo systems and installing them and that sort of thing. I have a large Kicker amp and anybody that knows Kicker knows they draw an ass of power from the battery. Well I want to put an extra battery in my trunk. I've already read the previous posts on how to wire it yadda yadda yadda. I just have a simple question...can't you just connect the positive of the 2 batteries and ground the other battery in the trunk instead of connecting negative and negative on the batteries? Or will that not allow the alternator to charge the second battery. Any help is appreciated....