TTTRRRRUUUSSSTTT MEEEEE.........unless the 2 batteries have the exact same amount of life in them and are installed at the same time, they'll drain each other. If you wired 2 news ones, you'll be alright is they are the EXACT same kind of battery, if anything is different then they will drain each other unless you get a battery isolator (makes alt. stop charging 1 after is done and works on the other one) or a celinoid(check spelling) and that physically separates the 2 battery's connection when the car turns off (unless it malfunctions).

AND ON TOP OF THAT

.....batteries have internal resistance , although very small (say 0.01ohms), when factored into the equation of Resistance= Volts divided by Current......that small ass 0.01 can make the differance of up to 4 or 5 volts sometimes.

I had a 460cc lead acid and a 950cc lead acid battery of my 140amp alternator wired to my 1200+ watt system before w/ a 2.4 farad cap......and my volts were at 11.7idleing and sometimes got as low as 10.5 durring continuous play. (that was the mustang)
Now in my Maxima i have one reblimished 650cc deep cycle (23 bucks at interstate battery) of my 120amp? 115? (not sure, but it's smaller) and i hold 14.9 idleing and 14.0 solid while bass is dropping pulling 1500watts rms. NO CAPACITOR

IMO from experience and reading about it car stereo magazine, unless you're going all out 3000+ watts rms i dont think you need an extra battery, if you're concerned get a nice optima or stinger and it'll do fine. my 23 dollar reblimmy is still pounding after 10 months of abuse......