Power Questions for Car Audio.?

[Tyler B]

Hey there guy's and gal's. I've a question about adding a second battary, i've read and researched ALOT and from what i've fount out that adding a second battary can mess up an alternator (Ive got a Aftermarket PowerMaster alternator pushing 140 AMPS) hooked up by 1 - 0/2 Guage wire and i've done the BIG THREE upgrade and have noticed a nice better look in my Volt meter (reads 14.4 at idle and when im Bumping the music LOUD it reads 14.1 ish) but im wanting to add a second battary, Could i use a Car Soloniod to seperate them when the car is off? or would i have to use one for Car Audio purposes? and im putting the 2nd battary in the trunk, would ONE run of 0/2 Gauge wire for both Posative and Negative be good enough? im only running a 1.200 Watt amp and a 300 Watt 4-channal and sometime down the line i might add another 300 watt 2 channal (Putting 6-1/2's in my rear window deck of my 89' Thunderbird) so, im just wondering...Thanks